2018: d05: ex2: add solution

2018/d05/ex2/ex2.py

@@ -0,0 +1,38 @@
+#!/usr/bin/env python
+
+import sys
+
+
+def solve(input: str) -> int:
+    def reduce_polymer(polymer: str) -> str:
+        for i in range(26):
+            lower, upper = chr(ord("a") + i), chr(ord("A") + i)
+            polymer = polymer.replace(lower + upper, "")
+            polymer = polymer.replace(upper + lower, "")
+        return polymer
+
+    def fully_reduced(polymer: str) -> str:
+        while True:
+            reduced = reduce_polymer(polymer)
+            if reduced == polymer:
+                return polymer
+            polymer = reduced
+        assert False  # Sanity check
+
+    def remove_defective(polymer: str, unit: str) -> str:
+        return polymer.replace(unit.lower(), "").replace(unit.upper(), "")
+
+    polymer = input.strip()
+    return min(
+        len(fully_reduced(remove_defective(polymer, chr(ord("a") + i))))
+        for i in range(26)
+    )
+
+
+def main() -> None:
+    input = sys.stdin.read()
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()