C# Stack and Queue: evaluating postfix expressions

Stack<T> is last-in, first-out with Push, Pop, and Peek. Queue<T> is first-in, first-out with Enqueue and Dequeue. Both operations are O(1). Popping from an empty stack or queue throws InvalidOperationException.

using System;
using System.Collections.Generic;

class Program {
  static int EvalPostfix(string expression) {
    var stack = new Stack<int>();
    foreach (string token in expression.Split(' ')) {
      if (int.TryParse(token, out int number)) {
        stack.Push(number);
      } else {
        int right = stack.Pop();
        int left = stack.Pop();
        stack.Push(token switch {
          "+" => left + right,
          "-" => left - right,
          "*" => left * right,
          "/" => left / right,
          _ => throw new ArgumentException("bad operator " + token),
        });
      }
    }
    return stack.Pop();
  }

  static void Main() {
    Console.WriteLine(EvalPostfix("3 4 + 2 *"));
    Console.WriteLine(EvalPostfix("5 1 2 + 4 * + 3 -"));
    try {
      EvalPostfix("2 3 ^");
    } catch (ArgumentException e) {
      Console.WriteLine(e.Message);
    }

    var line = new Queue<string>();
    line.Enqueue("Ada");
    line.Enqueue("Bo");
    line.Enqueue("Cy");
    Console.WriteLine(line.Dequeue() + " served, " + line.Count + " waiting, next is " + line.Peek());
  }
}

Read the full lesson