Longest Valid Parentheses

 Given a string containing just the characters '(' and ')', return 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

 


class Solution {
    public int longestValidParentheses(String s) {
       
        Stack<Integer> stack = new Stack<>();

        // Push -1 as a base for the first valid substring calculation
        stack.push(-1);

        int maxlength = 0;  // Stores the maximum length of valid parentheses

        // Iterate through the characters of the string
        for (int i = 0; i < s.length(); i++) {
            char ch = s.charAt(i);

            if (ch == '(') {
                // Push the index of '(' onto the stack
                stack.push(i);
            } else {
                // Pop the last index as we found a closing ')'
                stack.pop();

                if (stack.isEmpty()) {
                    // If stack is empty after popping, push current index as new base
                    stack.push(i);
                } else {
                    // Calculate the length of the current valid substring
                    int length = i - stack.peek();
                    maxlength = Math.max(maxlength, length);
                }
            }
        }

        return maxlength;
    }
}

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