feat: add swift implementation to lcof problem: No.64 (doocs#2959)

Author: klever34

lcof/面试题64. 求1+2+…+n/README.md

@@ -147,6 +147,18 @@ public class Solution {
 }
 ```
 
+#### Swift
+
+```swift
+class Solution {
+    func sumNums(_ n: Int) -> Int {
+        var s = n
+        let _ = n > 0 && { s += sumNums(n - 1); return true }()
+        return s
+    }
+}
+```
+
 <!-- tabs:end -->
 
 <!-- solution:end -->

lcof/面试题64. 求1+2+…+n/Solution.swift

@@ -0,0 +1,7 @@
+class Solution {
+    func sumNums(_ n: Int) -> Int {
+        var s = n
+        let _ = n > 0 && { s += sumNums(n - 1); return true }()
+        return s
+    }
+}