20. Valid Parentheses

题目描述【Leetcode】

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.

用栈来做就可以了

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