2018: d01: ex2: add solution

2018/d01/ex2/ex2.py

@@ -0,0 +1,31 @@
+#!/usr/bin/env python
+
+import itertools
+import sys
+
+
+def solve(input: str) -> int:
+    def parse(input: list[str]) -> list[int]:
+        return [int(n) for n in input]
+
+    def find_duplicate(deltas: list[int]) -> int:
+        start = 0
+        seen = {start}
+        for delta in itertools.cycle(deltas):
+            start += delta
+            if start in seen:
+                return start
+            seen.add(start)
+        assert False  # Sanity check
+
+    deltas = parse(input.splitlines())
+    return find_duplicate(deltas)
+
+
+def main() -> None:
+    input = sys.stdin.read()
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()