Leetcode: Evaluate Reverse Polish Notation

Question:

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

Thoughts:

A stack is needed to record operands and sub-results of the reverse polish notation

Scan the given postfix expression from left to right
for each symbol
 {
  if operand then
    push onto stack
  if operator then
   {
    operand1=pop stack
    operand2=pop stack
    compute operand1 operator operand2
    push result onto stack
   }
 }
return top of stack as result

If the question asks to evaluate a polish notation(prefix notation), do the same process as the post-fix expression, but scan the expression from right to left.

public class Solution {
    public int evalRPN(String[] tokens) {
        if (tokens.length <= 0) {
            return 0;
        }
        
        LinkedList<String> stack = new LinkedList<String>();
        String operators = "+-*/";
        for (String s : tokens) {
            if (!operators.contains(s)) {
                stack.push(s);
            } else {
                int num1 = Integer.parseInt(stack.pop());
                int num2 = Integer.parseInt(stack.pop());
                
                if (s.equals("+")) stack.push(new Integer(num1 + num2).toString());
                else if (s.equals("-")) stack.push(new Integer(num2 - num1).toString());
                else if (s.equals("*")) stack.push(new Integer(num1 * num2).toString());
                else if (s.equals("/")) stack.push(new Integer(num2 / num1).toString());
            }
        }
        
        return Integer.parseInt(stack.pop());
    }
}