2020: d09: ex2: add solution

2020/d09/ex2/ex2.py

@@ -0,0 +1,42 @@
+#!/usr/bin/env python
+
+
+import itertools
+import sys
+from typing import List
+
+
+def find_invalid(nums: List[int]) -> int:
+    for i in range(25, len(nums)):
+        num = nums[i]
+        found = False
+        for lhs, rhs in itertools.combinations(nums[i - 25 : i], 2):
+            if lhs + rhs == num:
+                found = True
+                break
+        if not found:
+            return num
+    assert False  # Sanity check
+
+
+def find_weakness(nums: List[int], invalid: int) -> int:
+    for i in range(len(nums) - 2):
+        for j in range(i + 2, len(nums)):
+            if sum(nums[i:j]) == invalid:
+                return min(nums[i:j]) + max(nums[i:j])
+    assert False  # Sanity check((
+
+
+def solve(raw: List[str]) -> int:
+    nums = [int(line) for line in raw]
+    invalid = find_invalid(nums)
+    return find_weakness(nums, invalid)
+
+
+def main() -> None:
+    input = [line.strip() for line in sys.stdin.readlines()]
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()