From d2abd31795bb7abfd455ccac4dd16de80d5faf19 Mon Sep 17 00:00:00 2001
From: Bruno BELANYI <bruno@belanyi.fr>
Date: Thu, 10 Dec 2020 07:29:28 +0100
Subject: [PATCH] 2020: d10: ex2: add solution

---
 2020/d10/ex2/ex2.py | 39 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 39 insertions(+)
 create mode 100755 2020/d10/ex2/ex2.py

diff --git a/2020/d10/ex2/ex2.py b/2020/d10/ex2/ex2.py
new file mode 100755
index 0000000..6af091e
--- /dev/null
+++ b/2020/d10/ex2/ex2.py
@@ -0,0 +1,39 @@
+#!/usr/bin/env python
+
+import sys
+from typing import Dict, List
+
+
+def make_chain(adapters: List[int]) -> int:
+    adapters += [0, max(adapters) + 3]
+    adapters = sorted(adapters)
+    jolts: Dict[int, int] = {}
+    N = len(adapters)
+
+    def rec(index: int) -> int:
+        if (res := jolts.get(index)) is not None:
+            return res
+        if index == N - 1:
+            return 1
+        total = sum(
+            rec(i)
+            for i in range(index + 1, min(N, index + 4))
+            if (adapters[i] - adapters[index]) <= 3
+        )
+        jolts[index] = total
+        return total
+
+    return rec(0)
+
+
+def solve(raw: List[str]) -> int:
+    return make_chain([int(line) for line in raw])
+
+
+def main() -> None:
+    input = [line.strip() for line in sys.stdin.readlines()]
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()