在文件中对前缀表达式求值，将答案写入另一个文件中

在文件中question.txt逐行存储：
add(23,45)
add(-20,sub(13,10))
add(sub(neg(4),12), muti(doubleMe(2),5))
div(54,add(3,sub(9,3))
将值写入文件answer.txt：
add(23,45)=68
add(20,sub(13,10)) = 23
add(sub(neg(4),12), muti(doubleMe(2),5)) = 4
div(54,add(3,sub(9,3)) = 6

#include <iostream>
#include <fstream>
#include <sstream>
#include <stack>

using namespace std;

int calculate(string expression) {
    stack<int> ops;
    stack<char> optrs;
    int operand1, operand2;
    
    for (int i = 0; i < expression.length(); i++) {
        if (expression[i] >= '0' && expression[i] <= '9') {
            ops.push(expression[i] - '0');  // 将数字字符转为 int 加入操作数栈
        } else if (expression[i] == '(') {
            optrs.push(expression[i]);      // 将左括号加入操作符栈 
        } else if (expression[i] == ')') { 
            while (optrs.top() != '(') {    // 将操作符栈中的操作符依次出栈并运算
                operand2 = ops.top(); ops.pop();
                operand1 = ops.top(); ops.pop();
                ops.push(calculate(operand1, operand2, optrs.top())); 
                optrs.pop();
            }
            optrs.pop();                    // 弹出左括号
        } else {
            while (!optrs.empty() && precedence(expression[i]) <= precedence(optrs.top())) { 
                operand2 = ops.top(); ops.pop();
                operand1 = ops.top(); ops.pop();
                ops.push(calculate(operand1, operand2, optrs.top()));
                optrs.pop();
            }
            optrs.push(expression[i]);      // 将当前操作符入栈
        }
    }
    
    while (!optrs.empty()) {
        operand2 = ops.top(); ops.pop();
        operand1 = ops.top(); ops.pop();
        ops.push(calculate(operand1, operand2, optrs.top()));
        optrs.pop();
    }
    return ops.top(); 
}

// 运算优先级比较
int precedence(char op) {
    if (op == '+' || op == '-') return 1;
    if (op == '*' || op == '/') return 2;
    return 0; 
}

// 进行运算
int calculate(int operand1, int operand2, char optr) {
    if (optr == '+') return operand1 + operand2;
    if (optr == '-') return operand1 - operand2;
    if (optr == '*') return operand1 * operand2;
    if (optr == '/') return operand1 / operand2;
}

int main() {
    ifstream infile("question.txt");
    ofstream outfile("answer.txt");
    
    string expression;
    while (getline(infile, expression)) {
        int result = calculate(expression);
        outfile << expression << "=" << result << endl;
    }
}