Question

1. What is wrong with the following code? This piece of code incorrectly attempts to remove all even values from a stack integers. How would you fix it? You will need to write a class with the main(String[] args) {….} method to test the following piece of code, find the problem, and fix it.

               while (!s1.isEmpty()){

                              int n = s1.pop();

                              if (n % 2 != 0) {

                                s1.push(n); // if odd put it back.

                              }

               }

Answer 1

/*
Given code forms an infinite loop, because s1 stack will never be empty if there are odd numbers in the stack
so, use a temporary stack move all odds to a different stack first
and then copy the temporary stack back into the original stack
 */
import java.util.Stack;

public class RemoveStackEvens {

    public static void main(String[] args) {
        Stack<Integer> s1 = new Stack<>();
        s1.push(2);
        s1.push(3);
        s1.push(9);
        s1.push(6);
        s1.push(7);
        s1.push(4);
        s1.push(5);
        System.out.println("Original stack: " + s1);
        Stack<Integer> temp = new Stack<>();
        while (!s1.isEmpty()) {
            int n = s1.pop();
            if (n % 2 != 0) {
                temp.push(n);
            }
        }
        while (!temp.isEmpty()) {
            s1.push(temp.pop());
        }
        System.out.println("Modified stack: " + s1);
    }
}