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.

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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;
    }
}