This is my first solution here, so be gentle :D. The issue was not with pop(), as pop was doing what it was supposed to. You forgot to add break points in your switch statement. It did every operation and added it to the stack giving it the illusion that pop was not working. I will leave the printing part to you. Cheers.
switch(op)
{
case '+':
stack.push((one + two) + "");
break;
case '-':
stack.push((one - two) + "");
break;
case '*':
stack.push((one * two) + "");
break;
case '/':
stack.push((one / two) + "");
break;
}