1 files changed, 32 insertions, 0 deletions

diff --git a/2024/09/day09a.cpp b/2024/09/day09a.cpp
new file mode 100644
index 0000000..c52c8f0
--- /dev/null
+++ b/2024/09/day09a.cpp
@@ -0,0 +1,32 @@
+#include <algorithm>
+#include <iostream>
+#include <map>
+#include <set>
+#include <sstream>
+#include <string>
+#include <string_view>
+#include <vector>
+using namespace std;
+
+int main() {
+        vector<long long> a;
+        char c;
+        for (int id = 0; cin >> c; id++)
+                for (int j = 0; j < c-'0'; j++)
+                        a.push_back(id % 2 == 0 ? id/2 : -1); // -1 = space
+
+        int j = 0;
+        for (int i = a.size()-1; i >= 0; i--) {
+                while (a[j] >= 0) j++;
+                if (a[j] == -2) break;
+                a[j] = a[i];
+                a[i] = -2; // -2 = freed up
+        }
+
+        long long checksum = 0;
+        for (long long i = 0; i < (long long)a.size() && a[i] != -2; i++)
+                checksum += i*a[i];
+        cout << checksum << endl;
+
+        return 0;
+}