Initial commit

Hard/longest-valid-parentheses/Solution.java

@@ -0,0 +1,21 @@
+class Solution {
+    public int longestValidParentheses(String str) {
+        Stack<Integer> stack = new Stack<>();
+        stack.push(-1);
+        
+        int max = 0;
+        for (int cur = 0; cur < str.length(); cur++) {
+            if (str.charAt(cur) == '(') {
+                stack.push(cur);
+            } else {
+                stack.pop();
+                if (stack.empty()) {
+                    stack.push(cur);
+                } else {
+                    max = Math.max(max, cur - stack.peek());
+                }
+            }
+        }
+        return max;
+    }
+}