Reverse Polish Notation - A Stack Application

Medium L.C. problem: https://leetcode.com/problems/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.

Note:

• Division between two integers should truncate toward zero.
• The given RPN expression is always valid. That means the expression would always evaluate to a result and there won't be any divide by zero operation.

Example 1:

Input: ["2", "1", "+", "3", "*"]
Output: 9
Explanation: ((2 + 1) * 3) = 9

Example 2:

Input: ["4", "13", "5", "/", "+"]
Output: 6
Explanation: (4 + (13 / 5)) = 6

Example 3:

Input: ["10", "6", "9", "3", "+", "-11", "*", "/", "*", "17", "+", "5", "+"]
Output: 22
Explanation: 
  ((10 * (6 / ((9 + 3) * -11))) + 17) + 5
= ((10 * (6 / (12 * -11))) + 17) + 5
= ((10 * (6 / -132)) + 17) + 5
= ((10 * 0) + 17) + 5
= (0 + 17) + 5
= 17 + 5
= 22

This is a classic application of Stack. Linear time, linear space. Use of Long just to avoid overflows. Code is below, cheers, ACC.

public class Solution
{
    public int EvalRPN(string[] tokens)
    {
        Stack stack = new Stack();

        foreach (string token in tokens)
        {
            string v1 = "";
            string v2 = "";
            long val = 0;
            switch (token)
            {
                case "+":
                    v1 = stack.Pop();
                    v2 = stack.Pop();
                    val = Int64.Parse(v2) + Int64.Parse(v1);
                    stack.Push(val.ToString());
                    break;
                case "-":
                    v1 = stack.Pop();
                    v2 = stack.Pop();
                    val = Int64.Parse(v2) - Int64.Parse(v1);
                    stack.Push(val.ToString());
                    break;
                case "*":
                    v1 = stack.Pop();
                    v2 = stack.Pop();
                    val = Int64.Parse(v2) * Int64.Parse(v1);
                    stack.Push(val.ToString());
                    break;
                case "/":
                    v1 = stack.Pop();
                    v2 = stack.Pop();
                    val = Int64.Parse(v2) / Int64.Parse(v1);
                    stack.Push(val.ToString());
                    break;
                default:
                    stack.Push(token);
                    break;
            }
        }

        return Int32.Parse(stack.Pop());
    }
}