今天又看了看这道题，解决了，原因是计算的时候，我原以为都是一位的数（即没有1+87这种的），现支持了多位数的运算。我的思路是在后缀表达式的两个数字之间用“.”来分隔开。
代码如下：

#include <iostream>
#include <string>
#include <stack>

using namespace std;

//操作符的栈内、外优先级
int isp(char x)//栈内
{
    if (x == '*' || x == '/') return 5;
    if (x == '+' || x == '-') return 3;
    if (x == '(') return 1;
    else return -1;
}
int icp(char x)//栈外
{
    if (x == '*' || x == '/') return 4;
    if (x == '+' || x == '-') return 2;
    if (x == '(') return 6;
    else return -1;
}

int main()
{
    //postfix的存储中，一个数字结束 用.来表示
    string infix, postfix;
    stack<char> operators;
    int T;
    cin >> T;
    char ch;
    for (int q = 0; q < T; ++q)
    {
        cin >> infix;
        for (int i = 0; infix[i] != '#'; i++)
        {
            ch = infix[i];
            if (ch >= '0' && ch <= '9')
            {//是操作数，输出
                postfix.push_back(ch);
            }
            else if (operators.empty())
            {//若是空的，让该操作符进栈
                operators.push(ch);
                postfix.push_back('.');
            }
            else if (ch == ')')
            {
                while (operators.top() != '(' && !operators.empty())
                {
                    postfix.push_back('.');
                    postfix.push_back(operators.top());
                    operators.pop();
                }
                operators.pop();
            }
            else if (ch == '(')
            {
                postfix.push_back('.');
                operators.push(ch);
            }
            else if (ch == '+' || ch == '-'
                || ch == '*' || ch == '/')
            {//是加减乘除
                postfix.push_back('.');
                while (isp(operators.top()) >= icp(ch))
                {
                    postfix.push_back(operators.top());
                    operators.pop();
                    if (operators.empty())
                    {
                        break;
                    }
                }
                operators.push(ch);
            }
        }
        while (!operators.empty())
        {
            postfix.push_back(operators.top());
            operators.pop();
        }
        //储存数字
        stack<long long int> re;
        //re顶的数字与正在读的数字是否属于一个数字
        bool iN = true;
        //开始进行计算
        char op;
        for (int y = 0; y < postfix.size(); ++y)
        {
            op = postfix[y];
            if ((op >= '0' && op <= '9'))
            {
                int l = op - '0';
                if (iN == true)
                {//是新的数字，直接入栈
                    re.push(l);
                    iN = false;
                }
                else
                {//不是新的数字
                    l += re.top() * 10;
                    re.pop();
                    re.push(l);
                }
            }
            else if (op == '.')
            {//前一个数字结束
                iN = true;
            }
            else
            {
                //出栈顶两个数字
                long long int a = re.top();
                re.pop();
                long long int b = re.top();
                re.pop();
                if (op == '+')
                {//加
                    re.push(a + b);
                    iN = true;
                }
                else if (op == '-')
                {//减
                    re.push(b - a);
                    iN = true;
                }
                else if (op == '*')
                {//乘
                    re.push(a * b);
                    iN = true;
                }
                else if (op == '/')
                {//除
                    re.push(b / a);
                    iN = true;
                }
            }
        }
        long long int result = re.top();
        cout << result << endl;
        infix.clear();
        postfix.clear();
    }
    return 0;
}