LeetCode(20)-Valid Parentheses

问题描述：

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.

问题分析：

直接用栈模拟，与栈顶元素相比较，比较简单。

解题方法：

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