LeetCode 20. 有效的括号

栈

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

map + stack

class Solution {
public:
    bool isValid(string s) {
        int length = s.length();
        if(length ==0) return true;
        if(length %2==1) return false;
        stack<char> stk;
        unordered_map<char,char> pairs={
            {')','('},
            {']','['},
            {'}','{'}
        };
        for(char ch:s){
            if(pairs.count(ch)){
                if(stk.empty() || stk.top()!=pairs[ch])
                    return false;
                stk.pop();
            }else{
                stk.push(ch);
            }
        }
        return stk.empty();
    }
};