Given a string containing just the characters '(' and ')', find the length of the longest valid (well-formed) parentheses substring.
For "(()", the longest valid parentheses substring is "()", which has length = 2.
Another example is ")()())", where the longest valid parentheses substring is "()()", which has length = 4.

public class Solution {
    public int longestValidParentheses(String s) {
        int len = s.length();
        if(len == 0){
            return 0;
        }
        
        int f[] = new int[len];
        int ret = 0;
        for(int i=1; i<len; i++){
            if(s.charAt(i) == ')'){
                if(i - f[i-1] - 1>=0 && s.charAt(i - f[i-1] - 1) == '('){
                    f[i] = f[i-1] + 2;
                    if(i - f[i] > 0){
                        f[i] += f[i-f[i]];
                    }
                    ret = Math.max(ret, f[i]);
                }
            }
        }
        return ret;
    }
}