Conversion of Infix expression to Postfix

Question

Below is the program i have written to convert a infix expression to postfix. It does give an output but its not always the right one. For example if we input the expression A+B*C-D/F+G , the expected output is ABC*+DF/G+- but rather the program outputs AB+C*D-F/G+. What is the problem in the program.

#include<iostream>
#include<stack>
#include<string.h>
using namespace std;
 
int prec(char oper){
    if(oper == '^')
      return 3;
    else if(oper == '*' || '/')
      return 2;
    else if(oper == '+' || '-')
      return 1;
    else
      return -1;
}
 
string itp(string s){
  stack<char> stack;
  string output = "";
  int num = s.length();
 
  for(int i = 0 ; i < num ; i++){
    char ch = s[i];
 
    if((ch>='a' && ch<='z')||(ch>='A' && ch<='Z')||(ch>='0' && ch<='9'))
      output = output + ch;
 
    else if(ch == '(')
      stack.push('(');
 
    else if(ch == ')'){
      while(stack.top()!='('){
        output = output + stack.top();
        stack.pop();
      }
      stack.pop();
    }
 
    else{
      while(!stack.empty() && prec(s[i]) <= prec(stack.top())){
        output = output + stack.top();
        stack.pop();
      }
      stack.push(ch);
    }
    }
  while (!stack.empty()) {
  output = output + stack.top();
  stack.pop();
  }
 
  return output;
}
 
 
int main(){
  string question;
  cout<<"Enter the infix expression : ";
    cin >> question;
    cout<<endl<<"Postfix expression : "<<itp(question);
    return 0;
}