JavaAlgorithms/Others/StackPostfixNotation.java

package Others;

import java.util.*;

public class StackPostfixNotation {
  public static void main(String[] args) {
    Scanner scanner = new Scanner(System.in);
    String post = scanner.nextLine(); // Takes input with spaces in between eg. "1 21 +"
    System.out.println(postfixEvaluate(post));
    scanner.close();
  }

  // Evaluates the given postfix expression string and returns the result.
  public static int postfixEvaluate(String exp) {
    Stack<Integer> s = new Stack<Integer>();
    Scanner tokens = new Scanner(exp);

    while (tokens.hasNext()) {
      if (tokens.hasNextInt()) {
        s.push(tokens.nextInt()); // If int then push to stack
      } else { // else pop top two values and perform the operation
        int num2 = s.pop();
        int num1 = s.pop();
        String op = tokens.next();

        if (op.equals("+")) {
          s.push(num1 + num2);
        } else if (op.equals("-")) {
          s.push(num1 - num2);
        } else if (op.equals("*")) {
          s.push(num1 * num2);
        } else {
          s.push(num1 / num2);
        }

        //  "+", "-", "*", "/"
      }
    }
    tokens.close();
    return s.pop();
  }
}