Solution.java

class Solution {
    public int longestValidParentheses(String s) {
        if (s == null || s.length() < 2) {
            return 0;
        }
        char[] chars = s.toCharArray();
        int n = chars.length;
        int[] res = new int[n];
        res[0] = 0;
        res[1] = chars[1] == ')' && chars[0] == '(' ? 2 : 0;
        
        int max = res[1];
        
        for (int i = 2; i < n; ++i) {
            if (chars[i] == ')') {
                if (chars[i - 1] == '(') {
                    res[i] = res[i - 2] + 2;
                } else {
                    int index = i - res[i - 1] - 1;
                    if (index >= 0 && chars[index] == '(') {
                        // ()(())
                        res[i] = res[i - 1] + 2 + (index - 1 >= 0 ? res[index - 1] : 0);
                    }
                }
            }
            max = Math.max(max, res[i]);
        }
        
        return max;
        
    }
}