BJP5 Exercise 16.7: deleteBack — Help me understand the solution

问题

I have been working on this exercise to practice ListNodes, and was very frustrated because even though I thought I wrote the code correctly (as shown below), it didn't let me pass.

public int deleteBack()
{
    ListNode p = front;
    if(p == null)
    {
        throw new NoSuchElementException(); 
    }

    if(p.next == null)
    {
        int data =  p.data;
        p = null; 
        return data;
    }

    while(p.next.next != null)
    {
        p = p.next;
    }
    int data = p.next.data;
    p.next = null;
    return data;
}

Next, I tried creating in total three new ListNodes equal to front. Although, I didn't quite see why this would be necessary.

public int deleteBack()
{
    ListNode p = front;
    if(p == null)
    {
        throw new NoSuchElementException(); 
    }

    ListNode q = front;
    if(q.next == null)
    {
        int data =  q.data;
        q = null; 
        return data;
    }

    ListNode r = front;
    while(r.next.next != null)
    {
        r = r.next;
    }
    int data = r.next.data;
    r.next = null;
    return data;
}

Unfortunately, that also gave me the same result as before (passing only three tests), until I changed q = null to front = null. After this change, all tests were passed.

What I'm trying to understand is

why my original code—which seems fine to me—doesn't work.
why I had to create more than one ListNode equal to front.
why I had to set front = null instead of q = null.

I'm far from being satisfied. Could someone help me understand why these changes were necessary?

回答1:

https://practiceit.cs.washington.edu/problem/view/bjp5/chapter16/e7-deleteBack
asks that private ListNode front; // null for an empty list

In your first case, front is already null (as its copy p is).
In your second case, you remove the last and only one value in the list, so front must become null as the list is now empty. Setting the local copy q to null doesn't take care of that.
In your 3rd case, there were more than 1 element in the list, so after removing the last element the list is not empty, and front must keep its not null value.

来源：https://stackoverflow.com/questions/58847642/bjp5-exercise-16-7-deleteback-help-me-understand-the-solution

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