leetcode刷题_OJ 20

Given a string containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid.

An input string is valid if:

1. Open brackets must be closed by the same type of brackets.
2. Open brackets must be closed in the correct order.

Note that an empty string is also considered valid.

这题的重点在于括号要合乎规范，包括一一对应，顺序也要按照{[(来排列。c++的栈用起来就很合适了，因为它的先进后出特性。

class Solution {
public:
    bool isValid(string s) {
        stack<char> res;
        //利用栈的特性，先进后出，所以一遇到左边的就先压入。然后遇到右边的，就到顶出栈顶元素，这样也符合{[()]}的排列顺序
        for(int i=0;i<s.size();i++){
            if(s[i] == '(' || s[i] == '[' || s[i] == '{'){
                res.push(s[i]);
            }
            else{
                if(res.empty()) return false;
                char top=res.top();
                res.pop();
                char relative;
                if(top == '(') relative=')';
                else if(top == '{') relative='}';
                else if(top == '[') relative=']';

                if(s[i] != relative) return false;
            }
        }
        if(!res.empty()) return false;
        return true;
    }
};