Create 1-Two-Sum.swift

Swift version of Two Sum. Accepted by Leetcode Judge.

Author: chandra9302

swift/1-Two-Sum.swift

@@ -0,0 +1,14 @@
+class Solution {
+    func twoSum(_ nums: [Int], _ target: Int) -> [Int] {
+        var prevMap = [Int:Int]() // val -> index
+        
+        for (i, n) in nums.enumerated() {
+            let diff = target - n
+            if let firstIndex = prevMap[diff] {
+                return [firstIndex, i]
+            }
+            prevMap[n] = i
+        }
+        return []
+    }
+}