150. Evaluate Reverse Polish Notation

[huimeich]

Evaluate the value of an arithmetic expression in Reverse Polish Notation.

Valid operators are +, -, *, /. Each operand may be an integer or another expression.

Some examples: ["2", "1", "+", "3", ""] -> ((2 + 1) \ 3) -> 9 ["4", "13", "5", "/", "+"] -> (4 + (13 / 5)) -> 6

[huimeich]

public class Solution {
    public int evalRPN(String[] tokens) {
        int res = 0;
        Stack<Integer> stack = new Stack<Integer>();
        for (String op : tokens){
            int opNum;
            if (op.equals("+")){
                res = stack.pop();
                res += stack.pop();
                stack.push(res);
            } else if (op.equals("-")){
                res = stack.pop();
                res = stack.pop() - res;
                stack.push(res);
            }else if (op.equals("*")){
                res = stack.pop();
                res *= stack.pop();
                stack.push(res);
            }else if (op.equals("/")){
                res = stack.pop();
                res = stack.pop() / res;
                stack.push(res);
            }else {
                stack.push(Integer.valueOf(op));
            }            
        }
        return stack.pop();
    }
}