20.Valid Parentheses (Stack)

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.

思路：与括号相关的问题，通常用栈来处理。需要考虑的是将什么信息入栈。本题将括号的类型入栈，在Longest Valid Parentheses中，是将左括号的位置入栈

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