25 files changed, 741 insertions, 0 deletions

diff --git a/01_introductory_problems/a.out b/01_introductory_problems/a.out
new file mode 100755
index 0000000..8c1bdbd
--- /dev/null
+++ b/01_introductory_problems/a.out
diff --git a/01_introductory_problems/apple_division_1623.cpp b/01_introductory_problems/apple_division_1623.cpp
new file mode 100644
index 0000000..fb367b4
--- /dev/null
+++ b/01_introductory_problems/apple_division_1623.cpp
@@ -0,0 +1,21 @@
+#include <iostream>
+#include <numeric>
+#include <vector>
+
+long long md(std::vector<long long>& a, long long p, long long t, size_t i) {
+        if (2*p >= t)
+                return 2*p - t;
+        if (i == a.size())
+                return t;
+        return std::min(md(a, p, t, i+1), md(a, p+a[i], t, i+1));
+}
+
+int main() {
+        size_t n;
+        std::cin >> n;
+        std::vector<long long> a(n);
+        for (size_t i = 0; i < n; i++)
+                std::cin >> a[i];
+        auto s = md(a, 0, std::accumulate(a.begin(), a.end(), (long long)0), 0);
+        std::cout << s << "\n";
+}
diff --git a/01_introductory_problems/bit_strings_1617.cpp b/01_introductory_problems/bit_strings_1617.cpp
new file mode 100644
index 0000000..6be46ac
--- /dev/null
+++ b/01_introductory_problems/bit_strings_1617.cpp
@@ -0,0 +1,10 @@
+#include <iostream>
+
+int main() {
+        constexpr long long M = 1e9+7;
+        long long n, a{1};
+        std::cin >> n;
+        while (--n >= 0)
+                a = (a * 2) % M;
+        std::cout << a << std::endl;
+}
diff --git a/01_introductory_problems/chessboard_and_queens_1624.cpp b/01_introductory_problems/chessboard_and_queens_1624.cpp
new file mode 100644
index 0000000..d595232
--- /dev/null
+++ b/01_introductory_problems/chessboard_and_queens_1624.cpp
@@ -0,0 +1,42 @@
+#include <bitset>
+#include <iostream>
+#include <string>
+
+void set_bit(std::bitset<64>& r, int i, int j) {
+        if (i >= 0 && i < 8 && j >= 0 && j < 8)
+                r |= 1ULL << (unsigned long long)(8*i + j);
+}
+
+std::bitset<64> maskall(const std::bitset<64>& b, int i, int j) {
+        std::bitset<64> r{b};
+        r |= 0x0101010101010101ULL << (unsigned long long)j;
+        for (int k = 0; k < 8; k++) {
+                set_bit(r, k, k+j-i);
+                set_bit(r, k, -k+j+i);
+        }
+        return r;
+}
+
+long long f(std::bitset<64>& b, int i) {
+        if (i == 8)
+                return 1;
+        long long s{0};
+        for (int j = 0; j < 8; j++) {
+                if (!b[8*i+j]) {
+                        auto rr = maskall(b, i, j);
+                        s += f(rr, i+1);
+                }
+        }
+        return s;
+}
+
+int main() {
+        std::bitset<64> b{0};
+        std::string s;
+        for (int i = 0; i < 8; i++) {
+                std::cin >> s;
+                for (int j = 0; j < 8; j++)
+                        b[8*i+j] = s[j] == '*';
+        }
+        std::cout << f(b, 0) << "\n";
+}
diff --git a/01_introductory_problems/coin_piles_1754.cpp b/01_introductory_problems/coin_piles_1754.cpp
new file mode 100644
index 0000000..c54cdf1
--- /dev/null
+++ b/01_introductory_problems/coin_piles_1754.cpp
@@ -0,0 +1,18 @@
+#include <algorithm>
+#include <iostream>
+
+bool f(int a, int b) {
+        auto max = std::max(a, b);
+        auto min = std::min(a, b);
+
+        return max <= 2*min && (2*min - max) % 3 == 0;
+}
+
+int main() {
+        int t, a, b;
+        std::cin >> t;
+        for (int i = 0; i < t; i++) {
+                std::cin >> a >> b;
+                std::cout << (f(a, b) ? "YES" : "NO") << std::endl;
+        }
+}
diff --git a/01_introductory_problems/creating_strings_1622.cpp b/01_introductory_problems/creating_strings_1622.cpp
new file mode 100644
index 0000000..689e083
--- /dev/null
+++ b/01_introductory_problems/creating_strings_1622.cpp
@@ -0,0 +1,30 @@
+#include <iostream>
+#include <string>
+#include <map>
+#include <vector>
+
+void gen(std::map<char, int>& a, int count,
+    std::string start, std::vector<std::string>& res) {
+        if (count == 0)
+                res.push_back(start);
+        for (auto [k, v] : a) {
+                if (v > 0) {
+                        a[k]--;
+                        gen(a, count - 1, start + k, res);
+                        a[k]++;
+                }
+        }
+}
+
+int main() {
+        std::string s;
+        std::map<char, int> a;
+        std::vector<std::string> sol;
+        std::cin >> s;
+        for (auto c : s)
+                a[c]++;
+        gen(a, s.size(), "", sol);
+        std::cout << sol.size() << "\n";
+        for (auto x : sol)
+                std::cout << x << "\n";
+}
diff --git a/01_introductory_problems/digit_queries_2431.cpp b/01_introductory_problems/digit_queries_2431.cpp
new file mode 100644
index 0000000..28c4a2f
--- /dev/null
+++ b/01_introductory_problems/digit_queries_2431.cpp
@@ -0,0 +1,23 @@
+#include <iostream>
+
+int f(long long k) {
+        long long n{1}, p{1}, d{1}, q{9};
+        while (p + d*q <= k) {
+                n += q;
+                p += d * q;
+                d++;
+                q *= 10;
+        }
+        long long x{(k-p) / d + n};
+        long long m{(k-p) % d};
+        for (long long j = 0; j < d-m-1; j++)
+                x /= (long long)10;
+        return x % (long long)10;
+}
+
+int main() {
+        long long k;
+        std::cin >> k;
+        while (std::cin >> k)
+                std::cout << f(k) << "\n";
+}
diff --git a/01_introductory_problems/gray_code_2205.cpp b/01_introductory_problems/gray_code_2205.cpp
new file mode 100644
index 0000000..4fa8a2d
--- /dev/null
+++ b/01_introductory_problems/gray_code_2205.cpp
@@ -0,0 +1,22 @@
+#include <iostream>
+#include <format>
+
+void print(int number, int ndigits) {
+        std::cout << std::format("{0:0{1}b}", number, ndigits) << "\n";
+}
+
+void print_all(int& start, int digit, int ndigits) {
+        if (digit == 0) {
+                print(start, ndigits);
+        } else {
+                print_all(start, digit-1, ndigits);
+                start ^= 1 << (digit-1);
+                print_all(start, digit-1, ndigits);
+        }
+}
+
+int main() {
+        int n, start{0};
+        std::cin >> n;
+        print_all(start, n, n);
+}
diff --git a/01_introductory_problems/grid_coloring_i_3311.cpp b/01_introductory_problems/grid_coloring_i_3311.cpp
new file mode 100644
index 0000000..b6c59a4
--- /dev/null
+++ b/01_introductory_problems/grid_coloring_i_3311.cpp
@@ -0,0 +1,17 @@
+#include <iostream>
+#include <string>
+
+int main() {
+        int n, m;
+        std::cin >> n >> m;
+        for (int i = 0; i < n; i++) {
+                std::string s;
+                std::cin >> s;
+                for (int j = 0; j < m; j++) {
+                        char c = 'A' + 2*((i+j)%2);
+                        c += c == s[j];
+                        std::cout << c;
+                }
+                std::cout << "\n";
+        }
+}
diff --git a/01_introductory_problems/grid_path_description_1625.cpp b/01_introductory_problems/grid_path_description_1625.cpp
new file mode 100644
index 0000000..b483168
--- /dev/null
+++ b/01_introductory_problems/grid_path_description_1625.cpp
@@ -0,0 +1,150 @@
+#include <algorithm>
+#include <bitset>
+#include <iostream>
+#include <queue>
+#include <string>
+#include <vector>
+
+/*
+The official solution uses the heuristic: if both adjacent squares
+in horizontal direction are visited or wall, and both in vertical direction
+are not visited (or the other way round), then we stop because we borked
+the square. I did not think of this criterion, so to check if the square
+is borked I do a full visit from the bottom-left corner. This is too slow,
+so I do this only at depths 10, 20, 30 and 40. This is good enough.
+*/
+
+class Tile {
+public:
+        int i;
+        int j;
+
+        bool valid() const { return i >= 0 && i < 7 && j >= 0 && j < 7; }
+        bool end() const { return i == 6 && j == 0; }
+        bool operator==(const Tile& t) const { return i == t.i && j == t.j; }
+        Tile u() const { return Tile{i-1, j}; }
+        Tile d() const { return Tile{i+1, j}; }
+        Tile l() const { return Tile{i, j-1}; }
+        Tile r() const { return Tile{i, j+1}; }
+        static Tile err() { return Tile{-1, -1}; }
+
+        Tile move(char c) const {
+                if (c == 'U') return u();
+                if (c == 'D') return d();
+                if (c == 'L') return l();
+                if (c == 'R') return r();
+                return err();
+        }
+
+        std::vector<Tile> neighbors() const {
+                return std::vector { u(), d(), l(), r() };
+        }
+};
+
+class Map {
+public:
+        Map() : b(), v(49, 4) {
+                for (int i = 0; i < 7; i++) {
+                        v[index(Tile{i, 0})]--;
+                        v[index(Tile{i, 6})]--;
+                        v[index(Tile{0, i})]--;
+                        v[index(Tile{6, i})]--;
+                }
+        }
+
+        bool visited(Tile t) const {
+                return !t.valid() || b.test(index(t));
+        }
+
+        void set(Tile t) {
+                if (!t.valid()) return;
+                b.set(index(t));
+                for (auto u : t.neighbors())
+                        if (u.valid())
+                                v[index(u)]--;
+        }
+
+        void reset(Tile t) {
+                if (!t.valid()) return;
+                b.reset(index(t));
+                for (auto u : t.neighbors())
+                        if (u.valid())
+                                v[index(u)]++;
+        }
+
+        std::vector<Tile> locked_neighbors(Tile t) const {
+                std::vector<Tile> r{};
+                for (auto u : t.neighbors())
+                        if (locked(u))
+                                r.push_back(u);
+                return r;
+        }
+
+        int count() const { return b.count(); }
+
+        bool borked() const {
+                std::bitset<49> vv{};
+                std::queue<Tile> q;
+                q.push(Tile{6, 0});
+                vv.set(index(Tile{6, 0}));
+                int c{1};
+                while (!q.empty()) {
+                        Tile t = q.front();
+                        q.pop();
+                        for (auto u : t.neighbors()) {
+                                if (!visited(u) && !vv.test(index(u))) {
+                                        vv.set(index(u));
+                                        c++;
+                                        q.push(u);
+                                }
+                        }
+                }
+
+                return c + count() < 49;
+        }
+
+private:
+        std::bitset<49> b;
+        std::vector<int> v;
+        static int index(Tile t) { return 7*t.i + t.j; }
+
+        bool locked(Tile t) const {
+                return t.valid() && !visited(t) && !t.end() && v[index(t)] < 2;
+        }
+};
+
+int f(Map& m, const std::string& s, size_t n, Tile t) {
+        if (n == 48) return t.end();
+        if (m.visited(t) || t.end()) return 0;
+
+        m.set(t);
+        if (n % 10 == 0 && m.borked()) {
+                m.reset(t);
+                return 0;
+        }
+
+        auto ln = m.locked_neighbors(t);
+
+        int r{0};
+        if (s[n] != '?') {
+                Tile nt = t.move(s[n]);
+                if (ln.size() == 0 || (ln.size() == 1 && ln[0] == nt))
+                        r = f(m, s, n+1, nt);
+        } else {
+                if (ln.size() == 0)
+                        r = f(m, s, n+1, t.u()) + f(m, s, n+1, t.d())
+                          + f(m, s, n+1, t.l()) + f(m, s, n+1, t.r());
+                if (ln.size() == 1)
+                        r = f(m, s, n+1, ln[0]);
+        }
+
+        m.reset(t);
+        return r;
+}
+
+int main() {
+        Map m;
+        std::string s;
+        std::cin >> s;
+        std::cout << f(m, s, 0, Tile{0, 0}) << "\n";
+}
diff --git a/01_introductory_problems/increasing_array_1094.cpp b/01_introductory_problems/increasing_array_1094.cpp
new file mode 100644
index 0000000..314c686
--- /dev/null
+++ b/01_introductory_problems/increasing_array_1094.cpp
@@ -0,0 +1,12 @@
+#include <algorithm>
+#include <iostream>
+
+int main() {
+        long long n, prev{0}, sum{0};
+        std::cin >> n;
+        while (std::cin >> n) {
+                sum += std::max<long long>(0, prev - n);
+                prev = std::max(prev, n);
+        }
+        std::cout << sum << std::endl;
+}
diff --git a/01_introductory_problems/knight_moves_grid_3217.cpp b/01_introductory_problems/knight_moves_grid_3217.cpp
new file mode 100644
index 0000000..212e82c
--- /dev/null
+++ b/01_introductory_problems/knight_moves_grid_3217.cpp
@@ -0,0 +1,40 @@
+#include <iostream>
+#include <queue>
+#include <tuple>
+#include <vector>
+
+struct Node {
+        int i;
+        int j;
+        int d;
+};
+
+int main() {
+        int n;
+        std::cin >> n;
+        std::vector<std::vector<int>> a(n, std::vector<int>(n, 1e7));
+        std::queue<Node> q;
+
+        q.push(Node{0, 0, 0});
+        while (!q.empty()) {
+                auto v = q.front();
+                q.pop();
+                if (v.i < 0 || v.j < 0 || v.i >= n || v.j >= n || a[v.i][v.j] <= v.d)
+                        continue;
+                a[v.i][v.j] = v.d;
+                q.push(Node{v.i-2, v.j-1, v.d+1});
+                q.push(Node{v.i-2, v.j+1, v.d+1});
+                q.push(Node{v.i-1, v.j-2, v.d+1});
+                q.push(Node{v.i-1, v.j+2, v.d+1});
+                q.push(Node{v.i+2, v.j-1, v.d+1});
+                q.push(Node{v.i+2, v.j+1, v.d+1});
+                q.push(Node{v.i+1, v.j-2, v.d+1});
+                q.push(Node{v.i+1, v.j+2, v.d+1});
+        }
+
+        for (auto& v : a) {
+                for (auto& x : v)
+                        std::cout << x << " ";
+                std::cout << "\n";
+        }
+}
diff --git a/01_introductory_problems/mex_grid_construction_3419.cpp b/01_introductory_problems/mex_grid_construction_3419.cpp
new file mode 100644
index 0000000..feb1171
--- /dev/null
+++ b/01_introductory_problems/mex_grid_construction_3419.cpp
@@ -0,0 +1,30 @@
+#include <algorithm>
+#include <bitset>
+#include <iostream>
+#include <vector>
+
+int firstzero(const std::bitset<200>& b) {
+        for (int i = 0; i < 200; i++)
+                if (!b.test(i))
+                        return i;
+        return -1;
+}
+
+int next(int i, int j, std::vector<std::bitset<200>>& col,
+    std::vector<std::bitset<200>>& row) {
+        int r = firstzero(row[i] | col[j]);
+        row[i][r] = col[j][r] = 1;
+        return r;
+}
+
+int main() {
+        int n;
+        std::cin >> n;
+        std::vector<std::bitset<200>> col(n), row(n);
+
+        for (int i = 0; i < n; i++) {
+                for (int j = 0; j < n; j++)
+                        std::cout << next(i, j, col, row) << " ";
+                std::cout << "\n";
+        }
+}
diff --git a/01_introductory_problems/missing_number_1083.cpp b/01_introductory_problems/missing_number_1083.cpp
new file mode 100644
index 0000000..527abb2
--- /dev/null
+++ b/01_introductory_problems/missing_number_1083.cpp
@@ -0,0 +1,9 @@
+#include <iostream>
+
+int main() {
+        long long n, m, sum{0};
+        std::cin >> n;
+        while (std::cin >> m)
+                sum += m;
+        std::cout << n*(n+1)/2 - sum << std::endl;
+}
diff --git a/01_introductory_problems/number_spiral_1071.cpp b/01_introductory_problems/number_spiral_1071.cpp
new file mode 100644
index 0000000..d4df631
--- /dev/null
+++ b/01_introductory_problems/number_spiral_1071.cpp
@@ -0,0 +1,27 @@
+#include <algorithm>
+#include <iostream>
+
+long long f(long long x, long long y) {
+        long long c = std::max(x, y);
+        if (c % 2) {
+                if (y >= x)
+                        return (c-1)*(c-1)+x;
+                else
+                        return c*c-y+1;
+        } else {
+                if (y >= x)
+                        return c*c-x+1;
+                else
+                        return (c-1)*(c-1)+y;
+        }
+}
+
+int main() {
+        int t;
+        std::cin >> t;
+        for (int i = 0; i < t; i++) {
+                long long x, y;
+                std::cin >> y >> x;
+                std:: cout << f(x, y) << std::endl;
+        }
+}
diff --git a/01_introductory_problems/palindrome_reorder_1755.cpp b/01_introductory_problems/palindrome_reorder_1755.cpp
new file mode 100644
index 0000000..8ebf4f4
--- /dev/null
+++ b/01_introductory_problems/palindrome_reorder_1755.cpp
@@ -0,0 +1,35 @@
+#include <iostream>
+#include <string>
+
+void printn(char c, int n) {
+        for (int i = 0; i < n; i++)
+                std::cout << c;
+}
+
+int main() {
+        std::string s;
+        int odd{0}, a['Z'+1] = {0};
+        char oddc{0};
+        std::cin >> s;
+        for (auto c : s)
+                a[(size_t)c]++;
+        for (char c = 'A'; c <= 'Z'; c++) {
+                if (a[(size_t)c] % 2) {
+                        odd++;
+                        oddc = c;
+                }
+        }
+        if (odd > 1) {
+                std::cout << "NO SOLUTION\n";
+        } else {
+                for (char c = 'A'; c <= 'Z'; c++)
+                        if (c != oddc)
+                                printn(c, a[(size_t)c]/2);
+                if (odd)
+                        printn(oddc, a[(size_t)oddc]);
+                for (char c = 'Z'; c >= 'A'; c--)
+                        if (c != oddc)
+                                printn(c, a[(size_t)c]/2);
+                std::cout << std::endl;
+        }
+}
diff --git a/01_introductory_problems/permutations_1070.cpp b/01_introductory_problems/permutations_1070.cpp
new file mode 100644
index 0000000..4438795
--- /dev/null
+++ b/01_introductory_problems/permutations_1070.cpp
@@ -0,0 +1,15 @@
+#include <iostream>
+
+int main() {
+        int n;
+        std::cin >> n;
+        if (n == 2 || n == 3) {
+                std::cout << "NO SOLUTION" << std::endl;
+        } else {
+                for (int i = 2; i <= n; i += 2)
+                        std::cout << i << " ";
+                for (int i = 1; i <= n; i += 2)
+                        std::cout << i << " ";
+                std::cout << std::endl;
+        }
+}
diff --git a/01_introductory_problems/raab_game_i_3399.cpp b/01_introductory_problems/raab_game_i_3399.cpp
new file mode 100644
index 0000000..bfa5193
--- /dev/null
+++ b/01_introductory_problems/raab_game_i_3399.cpp
@@ -0,0 +1,67 @@
+#include <iostream>
+#include <utility>
+#include <vector>
+
+class Game {
+public:
+        bool y;
+        std::vector<int> a;
+        std::vector<int> b;
+        int apts = 0;
+        int bpts = 0;
+
+        Game(int n, bool w)
+            : y{w}, a{std::vector<int>(n)}, b{std::vector<int>(n)} {}
+
+        void play(int i, int j) {
+                this->a[this->next] = i;
+                this->b[this->next] = j;
+                this->apts += i > j;
+                this->bpts += j > i;
+                this->next++;
+        }
+
+        friend std::ostream& operator<<(std::ostream& os, const Game g) {
+                if (!g.y) {
+                        os << "NO\n";
+                } else {
+                        os << "YES\n";
+                        for (auto x : g.a)
+                                os << x << " ";
+                        os << "\n";
+                        for (auto x : g.b)
+                                os << x << " ";
+                        os << "\n";
+                }
+                return os;
+        }
+private:
+        int next = 0;
+};
+
+Game play(int n, int a, int b) {
+        if (a + b > n)
+                return Game(1, false);
+
+        Game g(n, true);
+        for (int i = 0; i < n - (a+b); i++)
+                g.play(i+1, i+1);
+        for (int i = 0; i < a; i++)
+                g.play(n-a+i+1, n-(a+b)+i+1);
+        for (int i = 0; i < b; i++)
+                g.play(n-(a+b)+i+1, n-b+i+1);
+
+        if (g.apts != a || g.bpts != b)
+                g.y = false;
+
+        return g;
+}
+
+int main() {
+        int n, a, b, t;
+        std::cin >> t;
+        for (int i = 0; i < t; i++) {
+                std::cin >> n >> a >> b;
+                std::cout << play(n, a, b);
+        }
+}
diff --git a/01_introductory_problems/repetitions_1069.cpp b/01_introductory_problems/repetitions_1069.cpp
new file mode 100644
index 0000000..790f315
--- /dev/null
+++ b/01_introductory_problems/repetitions_1069.cpp
@@ -0,0 +1,20 @@
+#include <algorithm>
+#include <iostream>
+#include <string>
+
+int main() {
+        char cur{'x'};
+        int n{0}, m{0};
+        std::string str;
+        std::cin >> str;
+        for (auto c : str) {
+                if (c == cur) {
+                        n++;
+                } else {
+                        m = std::max(m, n);
+                        n = 1;
+                        cur = c;
+                }
+        }
+        std::cout << std::max(m, n) << std::endl;
+}
diff --git a/01_introductory_problems/string_reorder_1743.cpp b/01_introductory_problems/string_reorder_1743.cpp
new file mode 100644
index 0000000..d22041d
--- /dev/null
+++ b/01_introductory_problems/string_reorder_1743.cpp
@@ -0,0 +1,38 @@
+#include <algorithm>
+#include <array>
+#include <iostream>
+#include <iterator>
+#include <sstream>
+#include <string>
+
+int main() {
+        std::array<size_t, 26> a{};
+        std::string s;
+        std::cin >> s;
+        for (auto c : s) a[c-'A']++;
+
+        size_t i{0}, j{1}, tot{s.size()}, c{99};
+        std::stringstream ss{};
+        while (*std::max_element(a.begin(), a.end()) <= tot / 2) {
+                while (a[i] == 0) i++;
+                while (a[j] == 0 || j <= i) j++;
+                c = i == c ? j : i;
+                ss << (char)('A' + c);
+                a[c]--;
+                tot--;
+        }
+
+        j = std::distance(a.begin(), std::max_element(a.begin(), a.end()));
+        while (a[j] > 1) {
+                if (tot == a[j]) {
+                        std::cout << "-1\n";
+                        return 0;
+                }
+                while (a[i] == 0 || i == j) i++;
+                ss << (char)('A' + j) << (char)('A' + i);
+                a[i]--;
+                a[j]--;
+                tot -= 2;
+        }
+        std::cout << ss.str() << (char)('A' + j) << "\n";
+}
diff --git a/01_introductory_problems/tower_of_hanoi_2165.cpp b/01_introductory_problems/tower_of_hanoi_2165.cpp
new file mode 100644
index 0000000..2b83788
--- /dev/null
+++ b/01_introductory_problems/tower_of_hanoi_2165.cpp
@@ -0,0 +1,17 @@
+#include <iostream>
+
+void do_hanoi(int n, int l, int m, int r) {
+        if (n != 0) {
+                do_hanoi(n-1, l, r, m);
+                std::cout << l << " " << r << "\n";
+                do_hanoi(n-1, m, l, r);
+        }
+}
+
+int main() {
+        int n;
+        std::cin >> n;
+
+        std::cout << (1 << n) - 1 << "\n";
+        do_hanoi(n, 1, 2, 3);
+}
diff --git a/01_introductory_problems/trailing_zeros_1618.cpp b/01_introductory_problems/trailing_zeros_1618.cpp
new file mode 100644
index 0000000..04cf85b
--- /dev/null
+++ b/01_introductory_problems/trailing_zeros_1618.cpp
@@ -0,0 +1,9 @@
+#include <iostream>
+
+int main() {
+        int n, c{0};
+        std::cin >> n;
+        while (n > 0)
+                c += (n /= 5);
+        std::cout << c << std::endl;
+}
diff --git a/01_introductory_problems/two_knights_1072.cpp b/01_introductory_problems/two_knights_1072.cpp
new file mode 100644
index 0000000..15d5f55
--- /dev/null
+++ b/01_introductory_problems/two_knights_1072.cpp
@@ -0,0 +1,24 @@
+#include <iostream>
+#include <vector>
+
+int main() {
+        long long n, a;
+        std::vector<long long> b = {0, 0, 6, 28, 96};
+        std::cin >> n;
+
+        for (long long k = 1; k <= n; k++) {
+                if (k <= 4) {
+                        a = b[k];
+                } else {
+                        // Both knights in new strip
+                        a += (2*k - 1)*(k - 1) - 2;
+
+                        // One knight in new strip, one in previous square
+                        const long long s = (k-1)*(k-1);
+                        a += 5*(s - 2);
+                        a += 4*(s - 3);
+                        a += (2*k - 10)*(s - 4);
+                }
+                std::cout << a << std::endl;
+        }
+}
diff --git a/01_introductory_problems/two_sets_1092.cpp b/01_introductory_problems/two_sets_1092.cpp
new file mode 100644
index 0000000..4314654
--- /dev/null
+++ b/01_introductory_problems/two_sets_1092.cpp
@@ -0,0 +1,53 @@
+#include <iostream>
+#include <vector>
+
+void printarr(const std::vector<int>& a) {
+        std::cout << a.size() << std::endl;
+        for (auto x : a)
+                std::cout << x << " ";
+        std::cout << std::endl;
+}
+
+int main() {
+        int n;
+        std::cin >> n;
+
+        switch (n % 4) {
+        case 1:
+        case 2:
+                std::cout << "NO\n";
+                break;
+        case 0:
+                std::cout << "YES\n";
+                {
+                        std::vector<int> a(n/2), b(n/2);
+                        for (int i = 0; i < n/4; i++) {
+                                a[2*i] = 4*i+1;
+                                a[2*i+1] = 4*i+4;
+                                b[2*i] = 4*i+2;
+                                b[2*i+1] = 4*i+3;
+                        }
+                        printarr(a);
+                        printarr(b);
+                }
+                break;
+        case 3:
+                std::cout << "YES\n";
+                if (n == 3) {
+                        std::cout << "2\n1 2\n1\n3\n";
+                } else {
+                        std::vector<int> a(n/2+1), b(n/2);
+                        a[0] = 1; a[1] = 2; a[2] = 4; a[3] = 7;
+                        b[0] = 3; b[1] = 5; b[2] = 6;
+                        for (int i = 1; i < n/4; i++) {
+                                a[2*i+2] = 4*i+4;
+                                a[2*i+3] = 4*i+7;
+                                b[2*i+1] = 4*i+5;
+                                b[2*i+2] = 4*i+6;
+                        }
+                        printarr(a);
+                        printarr(b);
+                }
+                break;
+        }
+}
diff --git a/01_introductory_problems/weird_algorithm_1068.cpp b/01_introductory_problems/weird_algorithm_1068.cpp
new file mode 100644
index 0000000..35cbdfa
--- /dev/null
+++ b/01_introductory_problems/weird_algorithm_1068.cpp
@@ -0,0 +1,12 @@
+#include <iostream>
+
+int main() {
+        long long n;
+        std::cin >> n;
+
+        while (n != 1) {
+                std::cout << n << " ";
+                n = (n % 2) ? n * 3 + 1 : n / 2;
+        }
+        std::cout << 1 << std::endl;
+}