longest-valid-parentheses

【题目描述】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.

【考查内容】栈，字符串

//维护一个栈，里面保存'('的下标，每次进入一个')'时，栈弹出，ans更新为当前扫描的下标
//与栈顶元素之间的距离(因为中间可能有些括号已经被消掉了)，因为栈可能为空，所以需要
//记录下起始记录点last。
class Solution {
public:
    int longestValidParentheses(string s) {
        int ans=0,last=-1;
        stack st;
        for(int i=0;i