是否可以在循环中使用单个语句来反向链接列表？

Question

Assume we have

ListNode {
     ListNode next;
     int number;

     //gets sets
}

Is it possible to reverse the list using a loop with only one string whithin a loop?

Something like this:

for(...) {
// only one string of code (one statement)
}

One guy told me that it's possible but I don't understand how. Can you help with that please? Thanks!

Answer 1

Yes. It is possible. It isn't pretty.

First, a toString method for ListNode to verify the answer. You can print root and reverse(root) to see the results work.

public String toString() {
    if (next != null) {
        return "" + num + " " + next.toString();
    }
    return "" + num;
}

In-Place Reversal

A zero-line answer that reverses in place:

static ListNode reverse(ListNode node) {
    ListNode headNode = null;
    ListNode previousNode = null;
    for (; node != null; previousNode = headNode, headNode = node, node = node.next, headNode.next = previousNode)
        ;
    return headNode;
}

Basically, a lot of node child swapping. We track the previous node so we can attach it to the head node, but we also have to track the next node for iteration.

Return a new reversed List

Now, the code golfy reverse method that returns a new set of Nodes in a reverse order.

static ListNode reverse(ListNode node) {
    ListNode headNode = null;
    ListNode followerNode = null;
    for (; node != null; node = node.next, followerNode = headNode) {
        (headNode = new ListNode(node.num)).next = followerNode;
    }
    return headNode;
}

Basically, these methods abuse many things about Java that you really shouldn't. We abuse the for loop by not using any integers for iteration. Then we abuse the increment section by including two lines of code for assignment.

Then singular line of code sets the headNode to a new ListNode of the value, and then sets its next to the previous headNode .

Practically speaking, it would take less effort to write it correctly than to write something that saves lines.

I mean, technically, if you really wanted to, you could put everything into the for loop iterator portion and get no lines, but in the spirit of trying to actually follow the prompt, only true assignments have been placed in the for loop.

That being said, it is customary coding convention that we DO NOT stuff code into a for loop for execution to save a few lines. In this day and age, the only real purpose for that is shortening down spacing in code golf.

PS: in the future, if anyone tells you something is possible in Java you should probably ask them to explain it. Whether or not things are possible in Java is a superset of whether we should be doing it in Java.