Create LongestValidParentheses.java

Leetcode/LongestValidParentheses.java

@@ -0,0 +1,33 @@
+/**
+32. Longest Valid Parentheses
+Solved
+Hard
+Topics
+Companies
+Given a string containing just the characters '(' and ')', return the length of the longest valid (well-formed) parentheses 
+substring
+.
+
+ */
+class Solution {
+    public int longestValidParentheses(String s) {
+        Stack<Integer> stack = new Stack<>();
+        stack.push(-1);
+        int max_len = 0;
+
+        for (int i = 0; i < s.length(); i++) {
+            if (s.charAt(i) == '(') {
+                stack.push(i);
+            } else {
+                stack.pop();
+                if (stack.isEmpty()) {
+                    stack.push(i);
+                } else {
+                    max_len = Math.max(max_len, i - stack.peek());
+                }
+            }
+        }
+
+        return max_len;        
+    }
+}