feat: Add solution for LeetCode problem 268

Author: WhiteHyun

missing-number/WhiteHyun.swift

@@ -0,0 +1,13 @@
+//  
+//  268. Missing Number
+//  https://leetcode.com/problems/missing-number/description/
+//  Dale-Study
+//  
+//  Created by WhiteHyun on 2024/05/19.
+//  
+
+final class Solution {
+  func missingNumber(_ nums: [Int]) -> Int {
+    (0 ... nums.count).reduce(0, +) - nums.reduce(0, +)
+  }
+}