solve

Author: sonjh1217

missing-number/sonjh1217.swift

@@ -0,0 +1,9 @@
+class Solution {
+    // O(n) time / O(1) space
+    func missingNumber(_ nums: [Int]) -> Int {
+        let count = nums.count
+        var expectedSum = count * (count + 1) / 2
+        let actualSum = nums.reduce(0, +)
+        return expectedSum - actualSum
+    }
+}