Recursively remove all occurrences of an item in a linked list

Question

public static Node deleteAll(Node front, String target){
    if (front == null){ return null;}
    if (front.data.equals(target)){
        return deleteAll(front.next,target);
    }
    front.next=deleteAll(front.next,target);
    return front;
}

I'm trying to walk through this solution, but it is confusing me. Why doesn't it always result as null since at the end front will equal null.

Answer 1

When thinking about such problems it is a good idea to get a pen and paper and draw some stuff and think about it on a high level

For example
...............
Input
List: [3]-[2]-[5]-null
Target: 2
................

First call => Result

deleteAll(N[3], 2) => [3]
but next is now deleteAll(N[2], 2)
List = [3]-deleteAll(N[2], 2)

Second call

deleteAll(N[2], 2) => deleteAll(N[5], 2)
next node now skips the 2
List = [3]-deleteAll(N[5], 2)

Third call

deleteAll(N[5], 2) => [5]
but next is now deleteAll(null, 2)
List = [3]-[5]-deleteAll(null, 2)

Lat call returns null

List ends up clean with no 2s
List = [3]-[5]-null

Answer 2

You have three cases:

1. The original front node is null, so you return null.
2. The original front node holds target , so you discard front and return the result of the recursive call on the linked node.
3. The original front node does not hold target , so you perform the recursive call on the linked node and return front .

In number 1 you return null and in number 3 you return non-null. In number 2 you basically go again, and so return null or the next node. And so on.

This means it's possible to return null. But it's also possible to return non-null.