Question

Would appreciate any help with this. Am first year programming student and this is a homework assignment to look at recursion.
1. we had to build an arraylist recursively (done below and working for smallest problem and then higher)
2. test it using a harness (also below)
3. we then have to make a clone copy of the array list (which I think is working) and write a method to recursively reverse the arraylist and include it in the ListMethodRunner test; this is where I am stuck. I have included my code below which is brining up an error on ‘list = reverseList(list.remove(0));’ in the reverseList Method. Any suggestions where Im going wrong?

//List Methods
import java.util.*;

public class ListMethods
{
//new method that produces an array list of integers (tempList) based on input of int n
public static ArrayList<Integer> makeList(int n)
{
  ArrayList<Integer> tempList = null;
  if (n <= 0)  // The smallest list we can make
  {

      tempList = new ArrayList<Integer>(); // ceate the list tempList
      return tempList;                      //return blank list for this if statement

  }
  else        // All other size lists are created here
  {

      tempList = makeList(n-1); //recursively go through each variation of n from top down, when reach 0 will create the list
      tempList.add(n); // program will then come back up through the variations adding each value as it goes

  }
  return tempList; //return the tempList population with all the variations

   }

//create a copy of the values in the array list (tlist) and put it in (list)- used in next method
public static ArrayList<Integer> deepClone(ArrayList<Integer> tList)
{
   ArrayList<Integer> list = new ArrayList<Integer>();
   for (Integer i : tList)
   {
       list.add(new Integer(i));
    }
    return list;
}
//method that creates arraylist
   public static ArrayList<Integer> reverseList(ArrayList<Integer> tList)
  {
   ArrayList<Integer> list = ListMethods.deepClone(tList);
 if (list.size()<=1)    // The list is empty or has one element
   {
      return list;// Return the list as is – no need to reverse!
 }
 else
 {
   list = reverseList(list.remove(0)); //recurse through each smaller list 
                                        //removing first value until get to 1 and will create list above
   list.add(0);
  // Use the solution to a smaller version of the problem
 // to solve the general problem
 }
 return list;
 }
}

//List  Methods Runner


import java.util.ArrayList;

public class ListMethodsRunner
{
 public static void main(String[] args)
{
  ArrayList<Integer> tempList = ListMethods.makeList(100);
  if (tempList.size() == 0)
  {
      System.out.println("The list is empty");
  }
  else
  {
     for (Integer i : tempList)
     {
        System.out.println(i);
     }
  }

     }
}

Answer 1

Replace

list = reverseList(list.remove(0))

With

list.remove(0);
list = reverseList(list);

ArrayList::remove(int) returns the element that was removed from the list. (type Integer in this case)

reverseList needs an ArrayList<Integer> as parameter. Ergo the error.

You also have to store the element before inserting it again. Your code should look like this:

Integer num = list.remove(0);
list = reverseList(list); 
list.add(num);