添加 20. 有效的括号 Swift版本

problems/0020.有效的括号.md

@@ -283,6 +283,29 @@ 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
+}
+```
 
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