LeetCode Longest Valid Parentheses

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) {
if (s == null) {
return  0;
}
int maxlen=0;
Stack<Integer> stack = new Stack<Integer>();
stack.add(-1);
for (int i = 0; i < s.length(); i++) {
if (s.charAt(i) == '(') {
stack.add(i);
} else {
stack.pop();
if (stack.isEmpty()) {
stack.add(i);
} else {
maxlen=Math.max(maxlen,i-stack.peek());
}
}
}
return maxlen;
}
}