Add solution 20

Author: vulgur

20_ValidParentheses.swift

@@ -0,0 +1,27 @@
+class Solution {
+    func isValid(_ s: String) -> Bool {
+        var stack = [Character]()
+        for bracket in s {
+            stack.append(bracket)
+            if bracket == ")" || bracket == "}" || bracket == "]" {
+                let right = stack.popLast()!
+                if let left = stack.popLast() {
+                    if (left == "(" && right == ")") ||
+                    (left == "{" && right == "}") || 
+                    (left == "[" && right == "]") {
+                        continue
+                    } else {
+                        return false
+                    }
+                } else {
+                    return false
+                }
+            }
+        }
+        if stack.isEmpty {
+            return true
+        } else {
+            return false
+        }
+    }
+}