20.有效括号

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

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来源： https://www.cnblogs.com/fengmao31/p/15906695.html