Leetcode no. 32

32. 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) {
Stack<Integer> stack= new Stack<Integer>();
int max=0;
int left=-1;
for (int i = 0; i < s.length(); i++) {
if (s.charAt(i)=='(') stack.push(i);
else {
if (stack.isEmpty()) left=i;
else {
stack.pop();
if (stack.isEmpty()) max=Math.max(max, i-left);
else max= Math.max(max, i-stack.peek());
}
}
}
return max;
}
}