Added Longest Valid Parentheses Algorithm

Author: suhailmalik07

Dynamic-Programming/LongestValidParentheses.js

@@ -0,0 +1,43 @@
+/*
+  LeetCode -> https://leetcode.com/problems/longest-valid-parentheses/
+
+  Given a string containing just the characters '(' and ')',
+  find the length of the longest valid (well-formed) parentheses substring.
+*/
+
+const longestValidParentheses = (s) => {
+  const n = s.length
+  const stack = []
+
+  // storing results
+  const res = new Array(n).fill(-Infinity)
+
+  for (let i = 0; i < n; i++) {
+    const bracket = s[i]
+
+    if (bracket === ')' && s[stack[stack.length - 1]] === '(') {
+      res[i] = 1
+      res[stack[stack.length - 1]] = 1
+      stack.pop()
+    } else {
+      stack.push(i)
+    }
+  }
+
+  // summing all adjacent valid
+  for (let i = 1; i < n; i++) {
+    res[i] = Math.max(res[i], res[i] + res[i - 1])
+  }
+
+  // adding 0 if there are none so it will return 0 instead of -Infinity
+  res.push(0)
+  return Math.max(...res)
+}
+
+const main = () => {
+  console.log(longestValidParentheses(')()())')) // output -> 4
+  console.log(longestValidParentheses('')) // output -> 0
+  console.log(longestValidParentheses('(()')) // output -> 2
+}
+
+main()