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;
+    }
+}

Hard/longest-valid-parentheses/solution.cpp

@@ -0,0 +1,26 @@
+#include <stack>
+#include <string>
+
+class Solution {
+  public:
+    int longestValidParentheses(std::string line) {
+        std::stack<int> shelf;
+        shelf.push(-1);
+
+        int max = 0;
+        for (int i = 0; i < line.length(); ++i) {
+            if (line[i] == '(') {
+                shelf.push(i);
+            } else {
+                shelf.pop();
+                if (shelf.empty()) {
+                    shelf.push(i);
+                } else if (max < i - shelf.top()) {
+                    max = i - shelf.top();
+                }
+            }
+        }
+
+        return max;
+    }
+};