Longest Valid Parentheses

public class Solution {
public int longestValidParentheses(String s) {
if (s == null || s.length() < 2) {
return 0;
}

int length = s.length();
int result = 0;

int[] f = new int[length];

for (int i = length - 2; i >= 0; i--) {
if (s.charAt(i) == '(') {
int j = i + f[i + 1] + 1;
if (j < length && s.charAt(j) == ')') {
f[i] = f[i + 1] + 2;
if (j + 1 < length) {
f[i] += f[j + 1];
}
}
}
result = result > f[i] ? result : f[i];
}

return result;
}
}