Merge pull request youngyangyang04#917 from qxuewei/master

添加 20. 有效的括号 Swift版本

Author: youngyangyang04

problems/0020.有效的括号.md

@@ -283,6 +283,31 @@ var isValid = function(s) {
 };
 ```
 
+
+Swift
+```swift
+func isValid(_ s: String) -> Bool {
+    var stack = [String.Element]()
+    for ch in s {
+        if ch == "(" {
+            stack.append(")")
+        } else if ch == "{" {
+            stack.append("}")
+        } else if ch == "[" {
+            stack.append("]")
+        } else {
+            let top = stack.last
+            if ch == top {
+                stack.removeLast()
+            } else {
+                return false
+            }
+        }
+    }
+    return stack.isEmpty
+}
+```
+
 C:
 ```C
 //辅助函数：判断栈顶元素与输入的括号是否为一对。若不是，则返回False
@@ -324,6 +349,5 @@ bool isValid(char * s){
 }
 ```
 
-
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