[Stack] Add a solution to Longest Valid Parentheses

Stack/LongestValidParentheses.swift

@@ -0,0 +1,28 @@
+/**
+ * Question Link: https://leetcode.com/problems/longest-valid-parentheses/
+ * Primary idea: Push index to a stack and pop encountering ")"
+ * Time Complexity: O(n), Space Complexity: O(n)
+ */
+
+class LongestValidParentheses {
+    func longestValidParentheses(_ s: String) -> Int {
+        let sChars = Array(s.characters)
+        var stack = [Int]()
+        var longest = 0
+
+        for (i, char) in sChars.enumerated() {
+            if char == "(" || stack.isEmpty || sChars[stack.last!] == ")" {
+                stack.append(i)
+            } else {
+                let _ = stack.removeLast()
+                if stack.isEmpty {
+                    longest = max(longest, i + 1)
+                } else {
+                    longest = max(longest, i - stack.last!)
+                }
+            }
+        }
+
+        return longest
+    }
+}