Evaluate Reverse Polish Notation

很简单的stack，注意operation order

public class Solution {

    public int evalRPN(String[] tokens) {

        if(tokens ==null|| tokens.length==0) return 0;

        Stack<String> st = new Stack<String>();

        String opr = "+-*/";

        for(int i=0;i<tokens.length;i++){

            if(!opr.contains(tokens[i])){

                st.push(tokens[i]);

            }else{

                cal(st,tokens[i]);

            }

        }

        return Integer.valueOf(st.pop());

    }

    private void cal(Stack<String> st, String op){

        int x = Integer.valueOf(st.pop());

        int y = Integer.valueOf(st.pop());

        switch(op){

            case "+": st.push(String.valueOf(x+y));

                      break;

            case "-": st.push(String.valueOf(y-x));

                      break;

            case "*": st.push(String.valueOf(x*y));

                      break;  

            case "/": st.push(String.valueOf(y/x));

                      break;

        }

    }

}