Evaluate Reverse Polish Notation

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

Solution:

public class Solution {
  public int evalRPN(String[] a) {
    Stack<Integer> stack = new Stack<Integer>();
    for (int i = 0; i < a.length; i++) {
      switch (a[i]) {
        case "+":
          stack.push(stack.pop() + stack.pop());
          break;
        case "-":
          stack.push(-stack.pop() + stack.pop());
          break;
        case "*":
          stack.push(stack.pop() * stack.pop());
          break;
        case "/":
          int n1 = stack.pop(), n2 = stack.pop();
          stack.push(n2 / n1);
          break;
        default:
          stack.push(Integer.parseInt(a[i]));
      }
    }
    return stack.pop();
  }
}