solve problem

Author: delight010

missing-number/delight010.swift

@@ -0,0 +1,14 @@
+class Solution {
+    // Time O(n)
+    // Space O(1)
+    func missingNumber(_ nums: [Int]) -> Int {
+        var sum = 0
+        for i in 0..<nums.count {
+            sum += i
+            sum -= nums[i]
+        }
+        
+        return sum + nums.count
+    }
+}
+