Postfix Evaluation The reason to convert infix to postfix expression is that we

Question

Postfix Evaluation

The reason to convert infix to postfix expression is that we can compute the answer of postfix expression easier by using a stack.

•For example:

•For a postfix expression: 10 2 8 * + 3 -

•We can use stack to solve it

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Create a class with two methods:
•a method named : public static int postfixEvaluate(String s)
•Parameter s is the a postfix expression: all the numbers are int, operators are from +, -, / *,
•The method should return the results
•main method which is used to test the postfixEvaluate(String s)
•Test case:
•"6 8 2 / 1 - *“
•"8 5 * 7 4 2 + * +“
•"2 3 + 4 5 * +"

Explanation / Answer

import java.util.*;

public class Postfix_Main{

public static void main(String []args){
System.out.println(postfixEvaluate("6 8 2 / 1 - *")); // 18
System.out.println(postfixEvaluate("8 5 * 7 4 2 + * +")); // 82
System.out.println(postfixEvaluate("2 3 + 4 5 * +")); // 25
  
}

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());
           } else {
               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);
               }
              
           // "+", "-", "*", "/"
           }
       }
       return s.pop();
}
}

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