Hard-题目36：32. Longest Valid Parentheses

题目原文：
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.
题目大意：
给出一个括号字符串，求最长的合法匹配的子串的长度。
题目分析：
用一个栈维护，如果遇到左括号则把它的下标入栈，如果遇到右括号，则记录右括号和左括号间的长度记为curLen，并判断curlen是否大于最大值max，如果是则更新。总共扫描一遍就可以了。
源码：（language：cpp）

class Solution {
public:
 // Using a stack. One pass
    int longestValidParentheses(string s) {
        vector<int> stack;
        int maxLen = 0;
        for (int i = 0; i < s.size(); ++i)
        {
            if (s[i] == '(')
                stack.push_back(i);
            else {
                if (!stack.empty() && s[stack.back()] == '(') {
                    stack.pop_back();
                    int lastPos = -1;
                    if (!stack.empty())
                        lastPos = stack.back();
                    int curLen = i - lastPos;
                    maxLen = (maxLen < curLen) ? curLen : maxLen;
                } else
                    stack.push_back(i);
            }
        }
        return maxLen;
    }
};

成绩：
12ms,15.70%,8ms，43.95%
cmershen的碎碎念：
遇到表达式计算、括号匹配这类问题时，经常要考虑堆栈的特性。