31. 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) {
int maxLen = 0, last = -1;
Stack<Integer> lefts = new Stack<Integer>();
for (int i=0; i<s.length(); ++i) {
if (s.charAt(i)=='(') {
lefts.push(i);
} else {
if (lefts.isEmpty()) {
// no matching left
last = i;
} else {
// find a matching pair
lefts.pop();
if (lefts.isEmpty()) {
maxLen = Math.max(maxLen, i-last);
} else {
maxLen = Math.max(maxLen, i-lefts.peek());
}
}
}
}
return maxLen;
}
}