Add missing-number solution

Author: KyrieJ95

missing-number/joon.py

@@ -0,0 +1,11 @@
+from typing import List
+
+
+class Solution:
+    # Time: O(n)
+    # Space: O(1)
+    def missingNumber(self, nums: List[int]) -> int:
+        n = len(nums)
+        # MissingNumber = (Sum of 1, 2, ..., n) - Sum of nums)
+        # Time: O(n)
+        return n * (n + 1) // 2 - sum(nums)