I do not understand why I am seeing an index from my array list twice despite having removed it after the first occurrence

Question

I am trying to solve the Josephus problem with an array list. I noticed that even though I remove and index after it is killed it still shows up in my output.

Why does 2 appear again when it should have been removed?

Below is my current output:

There are 7 people in the circle.
1, 2, 3, 4, 5, 6, 7
2 
2, 4 
2, 4, 1 
2, 4, 1, 3 
2, 4, 1, 3, 2 
2, 4, 1, 3, 2, 0

You should sit in seat 4 if you want to survive!

public class project1 {
  public static int Josephus (int n, int k){
    ArrayList<Integer> circle = new ArrayList<Integer>();
    for (int p = 1; p <= n; p++) {
      circle.add(p);                                                              
    }
    System.out.println("There are " + n + " people in the circle.");
    System.out.println(circle);

    ArrayList<Integer> kill_order = new ArrayList<Integer>();
    for (int index=1; circle.size()!=1; index++){
      if (circle.size() > 1){
        index = (index + k - 1) % circle.size();
        kill_order.add(index);
        circle.remove(index);
        System.out.println(kill_order);
      } else if (circle.size()==1){
        System.out.println("Execution Order: " + kill_order + " ");
        index = 1;
      }
    }
    return circle.get(0);
  }

  public static void main(String[] args) {
    System.out.println("You should sit in seat " + Josephus(7, 2) + " if you want to survive!");
  }
}

Answer 1

There are two methods List.remove: remove(int index) will remove the item at the given index in the list, remove(Object o) will remove the first object equal to o.

I think in your code, the circle.remove(index); resolves to the first one, but you actually need the second one. See https://docs.oracle.com/javase/8/docs/api/java/util/List.html

This should fix that:

circle.remove((Integer)index);

Answer 2

You see the 2 the second time because you are adding the index of the person to "kill" instead of the value to the kill_order list.

this should work:

if (circle.size() > 1){
    index = (index + k - 1) % circle.size();
    kill_order.add(circle.get(index));
    circle.remove(index);
    System.out.println(kill_order);
}