LeetCode, Longest Valid Parentheses

LongestValidParentheses.java

public class Solution {
    public int longestValidParentheses(String s) {
        if (s == null || s.isEmpty()) {
            return 0;
        }    
        
        Stack<Pair> stack = new Stack<>();
        final int n = s.length();
        boolean [] b = new boolean [n];
        for (int i = 0; i < n; i++) {
            if (s.charAt(i) == ')' && !stack.isEmpty() && stack.peek().c == '(') {
                b[stack.pop().index] = b[i] = true;
            } else {
                stack.push(new Pair(i, s.charAt(i)));
            }
        }
        // System.out.println(Arrays.toString(b));
        
        int maxLen = 0, len = 0;
        for (int i = 0; i < n; i++) {
            if (b[i]) {
                len++;
            } else {
                maxLen = Math.max(maxLen, len);
                len = 0;
            }
        }
        maxLen = Math.max(maxLen, len);
        return maxLen;
    }
    
    class Pair {
        int index;
        char c;
        Pair (int i, char ch) {
            index = i;
            c = ch;
        }
    }
}