如何仅通过重新排列链接来重新排列 Java 中的 LinkedList 而不更改数据字段？

Question

So first of all, I just started learning about LinkedLists, so I'm sorry if I say something about it and it's totally wrong. I have an assignment where I have to write a method to switch the pairs of a LinkedList. If I have a list [3, 7, 4, 9], it would change to [7, 3, 9, 4] after the method. I'm not allowed to change any of the data fields of the nodes nor can I construct any new nodes. I'm trying to understand how LinkedLists work. So far this is what I have:

public void switchPairs() {
    if (front == null)
        return;
    ListNode current = head;
    ListNode next = head.next;
    current.next = null;

    next.next = current;

}

Right now I'm still trying to figure out how I can swap the first two values. My reasoning is that I can assign a new ListNode variable called current to the head, and then another new ListNode variable called next to head.next. Then I can assign the node after current to be null. Then that list is separate from the next variable that I created and I can assign the node after next to be current. But that doesn't work. So how can I go about doing this?

Answer 1

You need to follow as below.

1. While temp is not null && temp1 is not null
      1. Swap links of temp and temp1
      2. Move temp to temp1.next
      3. Move temp1 to temp.next if temp is not null 
                                 else make temp is null

swap two node links is as below. [switchPairs(temp, temp1)]

 1. newTemp = temp
 2. temp.link = temp1.link
 3. temp1.link = newTemp

Let's say

data|link is the pattern I am using here.

temp = 3|linkX 

temp1 = 5|linkY

newTemp = temp = Points to the address of Temp

temp.link = 3|linkY [temp.link = temp1.link]

temp1.link = 5|AdrOfTemp [temp1.link = newTemp]

If you still have doubts, please post your helper method which calls this function. I doubt that you may missed to switch the temp nodes.

Answer 2

Please refer the code below:

public void switchPairs() {
if (front == null)
    return;

ListNode current = head;
ListNode nextNode = head.next;
current.next = nextNode.next;
head = nextNode; 
head.next = current;

}

Hope this algorithm will help...

1. Assign the head node into the one temp node
2. Assign the head's next node into the another temp node
3. Replace the step 1's next node with the step 2's next node
4. Assign the head next node with the node created on step 1
5. Assign the head node with the node created on step 2

Answer 3

Since this is your school assignment, I am not giving you the complete solution. Rather, I'm writing solution to only the following requirement and I hope, you'll be able to complete the rest of your assignment with the help of this code.

Right now I'm still trying to figure out how I can swap the first two values.

public ListNode switchPairs(ListNode head) {
    if (head == null)
        return;
    ListNode current = head;
    head = current.getNext();
    current.setNext(head.getNext());
    head.setNext(current);
    return head;
}

Notes:

1. I have changed the return type from void to ListNode . You'll need to call it like

head = switchPairs(head);

from wherever (eg main ) you have created the linked list. Wherever you have created the linked list, you must have first created the head node and then added other nodes to it. After calling switchPairs , the head should point to the second node in the original linked list.

2. You should declare next as private and write public getter and setter for it as shown below:

public ListNode getNext() {
    return next;
} 
setNext(ListNode next) {
    this.next = next;
}

Note that your IDE can generate these two methods on a couple of clicks.