Question

Write a method named changeStack that takes in a Stack of Integer objects named stackIn as a parameter and returns another Stack of Integer objects. The returned Stack contains contents that is the result of switching the top half and the bottom half of the stackIn. But the ordering of integers in each half is NOT changed. In the case of odd-size stackIn, the middle element remains in the same position before and after the switch.

This method is OUTSIDE the class <E>.

Example 1:

                  stackIn                   Returned Stack

   top                                                               top     

                    30                                     100

                    10                                     50

                    100                                   30

   bottom    50                                     10        bottom

Example 2:

                  stackIn                   Returned Stack

   top                                                                top     

                    15                                     65

                    3                                       8

                    200                                   200

                  65                                     15     

bottom      8                                        3        bottom

The Stack class includes all methods necessary for the stack operations. You can consider Stack is like the ArrayDeque in Java API used as a Stack.

public static Stack<Integer> changeStack(Stack<Integer> stackIn)

Answer 1

public static Stack<Integer> changeStack(Stack<Integer> stackIn)
  {
      //checks if the size of stack is odd or even
      if(stackIn.size()%2==0)
      {
          //finds the half length of the stack
          int l=stackIn.size()/2;
          
          //creats two arrays to hold both half of stack       
          int arr[]=new int[l];
          int arr2[]=new int[l];
          
          //pop half stack and store in array
          for (int i = 0; i < l; i++) {
              arr[i]=stackIn.pop();
          }
          //pop second half stack and store in array
          for (int i=0;i<l;i++) {
              arr2[i]=stackIn.pop();
          }
          
          //push the first array elements back into the stack one by one in the reverse order 
          for (int i = l-1; i>=0; i--) {
              stackIn.push(arr[i]);
          }

          //push the second array elements back into the stack one by one in the reverse order 
          for (int i = l-1; i>=0; i--) {
              stackIn.push(arr2[i]);
          }
          
          
      }
      else
      {
          int l=stackIn.size()/2;
          
          int arr[]=new int[l];
          int arr2[]=new int[l];
          
          for (int i = 0; i < l; i++) {
              arr[i]=stackIn.pop();
          }
          
          //size of the stack is odd so the next element after popping half the array 
          //must be the middle element. Store it separately in an integer variable
          int middleElement=stackIn.pop();
          
          for (int i=0;i<l;i++) {
              arr2[i]=stackIn.pop();
          }
          
          for (int i = l-1; i>=0; i--) {
              stackIn.push(arr[i]);
          }
          
          //after storing the half elements back in the array, we mush push the middle element
          //as its position should remain unchanged
          stackIn.push(middleElement);
          
          
          for (int i = l-1; i>=0; i--) {
              stackIn.push(arr2[i]);
          }
      }
      return stackIn;
}

Refer to the screenshot below for better clarity and indentation: