LeetCode 32. Longest Valid Parentheses (javascript)

Given a string containing just the characters '(' and ')', find the length of the longest valid (well-formed) parentheses substring.

Example 1:

Input: s = "(()"
Output: 2
Explanation: The longest valid parentheses substring is "()".

Example 2:

Input: s = ")()())"
Output: 4
Explanation: The longest valid parentheses substring is "()()".

Example 3:

Input: s = ""
Output: 0

Constraints:

0 <= s.length <= 3 * 10⁴
s[i] is '(', or ')'.

Solution:

/**
 * @param {string} s
 * @return {number}
 */
var longestValidParentheses = function(s) {
    let stack = [];
    // track the start position
    let start = 0;
    let ans = 0;
    
    for (let i = 0; i < s.length; i++) {
        // open parenthese '(' push that index of '('
        if (s[i] === '(') {
            stack.push(i);
        } else {
            // if stack empty => invalid, start from i + 1
            if (stack.length === 0) {
                start = i + 1;
            } else {
                stack.pop();
                // empty stack: calculate [start...i]  
                // or Not empty stack calculate [index of '('...i]
                // store the max length
                ans = Math.max(ans, stack.length === 0 ? i - start + 1 : i - stack[stack.length - 1]);
            }
        }
    }
    return ans;
};