Update and rename Solution.swift to Solution2.swift

Author: yanglbme

lcci/17.04.Missing Number/Solution.swift

@@ -0,0 +1,6 @@
+class Solution {
+    func missingNumber(_ nums: [Int]) -> Int {
+        let n = nums.count
+        return n * (n + 1) / 2 - nums.reduce(0, +)
+    }
+}