[LeetCode]Longest Valid Parentheses

题目描述:(链接）

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.

题目解析：

用栈！

class Solution {
public:
    int longestValidParentheses(string s) {
        vector<int> cache;
        int last = -1; //最后一个'）'
        int max_len = 0;
        for (int i = 0; i < s.size(); ++i) {
            if (s[i] == ')') {
                if (cache.empty()) {
                    last = i;
                } else {
                    cache.pop_back();
                    if (cache.empty()) {
                        max_len = max(max_len, i - last);
                    } else {
                        max_len = max(max_len, i - cache.back());
                    }
                }
            } else {
                cache.push_back(i);
            }
        }
        return max_len;
    }
};