leetcode 之 Valid Parentheses

题目如下：

Given a string containing just the characters ‘(‘, ‘)’, ‘{‘, ‘}’, ‘[‘ and ‘]’, determine if the input string is valid.
The brackets must close in the correct order, “()” and “()[]{}” are all valid but “(]” and “([)]” are not.

本题很容易想到用到栈，对于给定的字符串，遍历其每一个元素，如果遇到 ‘(’ ‘[’ ‘{’ 就将他们压入栈中，如果遇到 ‘)’ ‘]’ ‘}’，就取栈顶元素，看是否与其匹配，如果匹配则将栈顶元素pop掉，如果不匹配，返回false；遍历到最后检查栈是否为空，如果为空则为true，否则则为false；具体实现如下：

class Solution {public:    bool isValid(string s) {        int n = s.size();        stack<char> a;        for (int i = 0; i < n; i++) {            if(s[i] == '(' || s[i] == '[' || s[i] == '{') {                a.push(s[i]);            }            if(s[i] == ')') {                if(!a.empty() && a.top() == '(') {                    a.pop();                }                else return false;            }            if(s[i] == '}') {                if(!a.empty() && a.top() == '{') {                    a.pop();                }                else return false;            }            if(s[i] == ']') {                if(!a.empty() && a.top() == '[') {                    a.pop();                }                else return false;            }        }        return a.empty();    }};