ACM：括号配对问题

#include<iostream>
#include<string>
#include<stack>
using namespace std;

bool check(string str)
{
    stack<char> stack;
    for (int i = 0; i < str.length(); ++i)
    {
        if (str[i] == ‘(‘ || str[i] == ‘[‘ || str[i] == ‘{‘)
            stack.push(str[i]);                                    //左符入栈
        else if (stack.empty())                                    //非左符同时栈内无右符，则配对失败
            return false;
        else
            if (!(stack.top() == ‘(‘ && str[i] == ‘)‘ ||           //栈顶与当前符号不配对，则配对失败
                stack.top() == ‘[‘ && str[i] == ‘]‘ ||
                stack.top() == ‘{‘ && str[i] == ‘}‘))
                return false;
            else
                stack.pop();                                       //出栈，配对成功
    }
    return stack.empty();                                          //空栈，配对成功
}

int main()
{
    int n = 0;
    cin >> n;
    for (int i = 0; i < n; ++i)                                    //多次测试
    {
        string str;
        cin >> str;
        cout << (check(str)?"Yes":"No") << endl;
    }
}