İnfix to Postfix Expression Evaluator cpp

Question

This is all about stacks.We should use 2 stacks for this. I would like to convert infix expression to postfix expression and evaluate this with 2 function like, string Infix2Postfix(string& s) and int EvaluatePostfixExpression(string& s).The operators +, -, *, /, (, ), and follow the usual precedence rules: * and / has the highest precedence, + and - come next, and ( has the lowest precedence. My code isnt working ı really really need help!

    #include <iostream>
    #include <stack>
    #include <string>
    
    using namespace std;
    
    bool isOperator(char ch) {
        return ch == '+' || ch == '*';
    }
    int precedence(char op) {
        return op == '+' ? 1 : 2;
    }
    bool shouldReduce(stack<char>& stack, char op) {
        return !stack.empty() && precedence(stack.top()) >= precedence(op);
    } 
    string Infix2Postfix(string& s) {
        string postfix;
        stack<char>stack;
        for (int i = 0; i < s.size(); i++) {
            if (isdigit(s[i])) {
                postfix += s[i];
            }
            else if (isOperator(s[i])) {
                while (shouldReduce(stack, s[i]) && stack.top() != '(')
                    postfix += stack.top();
                stack.pop();
                stack.push(s[i]);
            }
            else if (s[i] == '(') {
                stack.push(s[i]);
            }
            else { // infix[i] == ')'
                while (stack.top() != '(') {
                 postfix.push_back(stack.top());
                 stack.pop();                      
                }
            }
            while (!stack.empty()) {
                postfix.push_back(stack.top());
                stack.pop();         
            }
        }
        string result;
        result = postfix;
        return result;
    }

// main function
#include <stdio.h>
#include <string>

using namespace std;

extern string Infix2Postfix(string& s);
extern int EvaluatePostfixExpression(string& s);
int Test1() {
    // infixExpr: 3500 - ((43*12) + (47/2));    
    // Let the C++ compiler do the evaluation :-)
    int expressionValue = 3500 - ((43 * 12) + (47 / 2));

    // postfix: 3500 43 12 * 47 2 / + -
    string postfixExpr = "3500 43 12 * 47 2 / + -";
    int result = EvaluatePostfixExpression(postfixExpr);
    if (result != expressionValue) return 0;

    return result;
} //end-Test1

/****************************************************
 * Test2
 ****************************************************/
int Test2() {
    int expressionValue = 20 + 2 * 3 + (2 * 8 + 5) * 4;

    string infixExpr = "20 + 2 * 3     + (2*8 + 5)* 4";
    string postfixExpr = Infix2Postfix(infixExpr);
    printf("Infix Expr: <%s>\nPostfixExpr: <%s>\n", infixExpr.c_str(), postfixExpr.c_str());

    int result = EvaluatePostfixExpression(postfixExpr);
    if (result != expressionValue) return 0;

    return result;
} //end-Test2

/****************************************************
 * Test3
 ****************************************************/
int Test3() {
    // Let the C++ compiler do the evaluation :-)
    int expressionValue = 20 * 2 + 3 - (2 * 8 + 5) * 4;

    string infixExpr = "20* 2 + 3 - (2*8 + 5)* 4";
    string postfixExpr = Infix2Postfix(infixExpr);
    printf("Infix Expr: <%s>\nPostfixExpr: <%s>\n", infixExpr.c_str(), postfixExpr.c_str());

    int result = EvaluatePostfixExpression(postfixExpr);
    if (result != expressionValue) return 0;

    return result;
} //end-Test3

/****************************************************
 * Test4
 ****************************************************/
int Test4() {
    int expressionValue = 220 - 45 - 10;

    string infixExpr = "220 - 45 - 10";
    string postfixExpr = Infix2Postfix(infixExpr);
    printf("Infix Expr: <%s>\nPostfixExpr: <%s>\n", infixExpr.c_str(), postfixExpr.c_str());

    int result = EvaluatePostfixExpression(postfixExpr);
    if (result != expressionValue) return 0;

    return result;
} //end-Test4

/****************************************************
 * Test5
 ****************************************************/
int Test5() {
    int expressionValue = (((13 + 35) * 22) / 45) - (45 + 34 * (190 - 34)) / 100;

    string infixExpr = "(((13+35)*22)/45) - (45+34*(190-34))/100";
    string postfixExpr = Infix2Postfix(infixExpr);
    printf("Infix Expr: <%s>\nPostfixExpr: <%s>\n", infixExpr.c_str(), postfixExpr.c_str());

    int result = EvaluatePostfixExpression(postfixExpr);
    if (result != expressionValue) return 0;

    return result;
} //end-Test5