gistfile1.java

public class Solution {
    interface Operation {public int op(int a, int b);}
    class Add implements Operation{public int op(int a, int b){return a+b;}}
    class Sub implements Operation{public int op(int b, int a){return a-b;}}
    class Mul implements Operation{public int op(int a, int b){return a*b;}}
    class Div implements Operation{public int op(int b, int a){return a/b;}}
    public Operation getOp(String t) {
        switch(t){
            case "+": return new Add();
            case "-": return new Sub();
            case "*": return new Mul();
            case "/": return new Div();
            default: return null;
        }
    }
    public int evalRPN(String[] tokens) {
        Stack<Integer> s = new Stack<Integer>();
        for(String t : tokens) {
            Operation o = getOp(t);
            if(o != null){
                s.push(o.op(s.pop(), s.pop()));
            } else {
                s.push(Integer.parseInt(t));
            }
        }
        return s.pop();
    }
}