compute post expression

#include<iostream>
#include<stack>
#include<string>
using namespace std;//post expression compute
int compute(int num1, int num2, char op)
{
if (op == '+')
return num1 + num2;
else if (op == '-')
return num1 - num2;
else if (op == '*')
return num1*num2;
else if (op == '/')
return num2 / num1;
}int main()
{
stack<char>num;
string s;
cin >> s;for (int i = 0; i<s.length(); i++)
{
if ((s[i] == '0' || s[i] == '1' || s[i] == '2' || s[i] == '3' || s[i] == '4' || s[i] == '5' || s[i] == '6' || s[i] == '7' || s[i] == '8' || s[i] == '9'))
num.push(s[i]);
else
{
int num1 = num.top() - '0';
num.pop();
int num2 = num.top() - '0';
num.pop();
char result = compute(num1, num2, s[i]) + '0';
num.push(result);
}
}cout << num.top()-'0' << endl;return 0;
}