算法學習--棧--Longest Valid Parentheses

題目

Given a string containing just the characters '(' and ')', find the length of the longest valid (wellformed) 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.

class Solution {
public:
    int longestValidParentheses(const string& s) {
        int max_len = 0, last = -1; // the position of the last ')'
        stack<int> lefts; // keep track of the positions of non-matching '('s
        for (int i = 0; i < s.size(); ++i) {
            if (s[i] =='(') {
            lefts.push(i);
            } else {
                if (lefts.empty()) {
                    // no matching left
                    last = i;
                } else {
                    // find a matching pair
                    lefts.pop();
                    if (lefts.empty()) {
                        max_len = max(max_len, i-last);
                    } else {
                        max_len = max(max_len, i-lefts.top());
                    }
                }
            }
        }
        return max_len;
    }
};