Clean up and alternative solution to day 7

4 files changed, 23 insertions, 2 deletions

diff --git a/2025/07/a.py b/2025/07/a.py
index 0e13b75..53ff68c 100644
--- a/2025/07/a.py
+++ b/2025/07/a.py
@@ -2,9 +2,9 @@ import fileinput
 
 with fileinput.input() as lines:
         rows = [line[:-1] for line in lines]
-        pos = [rows[0].find('S')]
 
 s = 0
+pos = [rows[0].find('S')]
 for row in rows[1:]:
         newpos = set()
         for p in pos:
diff --git a/2025/07/b.py b/2025/07/b.py
index 41c0db0..4457654 100644
--- a/2025/07/b.py
+++ b/2025/07/b.py
@@ -2,8 +2,8 @@ import fileinput
 
 with fileinput.input() as lines:
         rows = [line[:-1] for line in lines]
-        pos = {rows[0].find('S'): 1}
 
+pos = {rows[0].find('S'): 1}
 for row in rows[1:]:
         newpos = {}
         for p, w in pos.items():
diff --git a/2025/07/b2.py b/2025/07/b2.py
new file mode 100644
index 0000000..b77188c
--- /dev/null
+++ b/2025/07/b2.py
@@ -0,0 +1,17 @@
+import fileinput
+
+with fileinput.input() as lines:
+        rows = [line[:-1] for line in lines]
+
+t = [1 if x == 'S' else 0 for x in rows[0]]
+for row in rows[1:]:
+        next = [0] * len(row)
+        for i in range(len(row)):
+                if row[i] == '^':
+                        next[i-1] += t[i]
+                        next[i+1] += t[i]
+                else:
+                        next[i] += t[i]
+        t = next
+
+print(sum(t))
diff --git a/2025/README.md b/2025/README.md
index c859ec8..4ca1d2e 100644
--- a/2025/README.md
+++ b/2025/README.md
@@ -96,3 +96,7 @@ changed to a map where the keys are the positions and the values are the
 number of multiverses where a tachyon is in that position.  To update
 this value for the current row, I sum the values of all tachyons that
 end there from the previous row (that can be one or two tachyons).
+
+I added a second solution for part 2 that does not use a map, but only
+lists. This could be seen as a dynamic programming problem where the
+iterative implementation is more intuitive than the recursive one.