2021: d12: ex2: add solution

2021/d12/ex2/ex2.py

@@ -0,0 +1,49 @@
+#!/usr/bin/env python
+
+import itertools
+import sys
+from collections import defaultdict
+from typing import Dict, List, Set
+
+Map = Dict[str, Set[str]]
+
+
+def solve(input: List[str]) -> int:
+    def parse() -> Map:
+        res: Map = defaultdict(set)
+
+        for (start, to) in map(lambda s: s.split("-"), input):
+            res[start].add(to)
+            res[to].add(start)
+
+        return res
+
+    caves = parse()
+
+    def dfs(start: str, seen: Set[str] = set(), has_doubled_back: bool = False) -> int:
+        if start == "end":
+            return 1
+
+        seen = seen | {start}
+        res = 0
+
+        for dest in caves[start]:
+            doubles_back = False
+            if dest in seen and dest.islower():
+                if has_doubled_back or dest == "start":
+                    continue
+                doubles_back = True
+            res += dfs(dest, seen, has_doubled_back or doubles_back)
+
+        return res
+
+    return dfs("start")
+
+
+def main() -> None:
+    input = [line.strip() for line in sys.stdin.readlines()]
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()