24. Swap Nodes in Pairs (Medium)

https://leetcode.com/problems/swap-nodes-in-pairs/

Given a linked list, swap every two adjacent nodes and return its head.

You may not modify the values in the list's nodes, only nodes itself may be changed.

Example:

Given 1->2->3->4, you should return the list as 2->1->4->3.

Solutions

class Solution {

    // To make it easy, it is strongly recommended to prepend a dummy node as the 'head' of the list.

    public ListNode swapPairs(ListNode head) {
        if (head == null) {
            return null;
        }

        if (head.next == null) {
            return head;
        }

        // Create a dummy head node to lead the list, which facilitates the swapping operation.
        ListNode dummy = new ListNode(0);
        dummy.next = head;

        // p1->p2->p3; p2 and p3 are nodes being swapped
        ListNode p1 = dummy;
        ListNode p2 = p1.next;
        ListNode p3 = p2.next;

        // examples: -1->1->2->3; -1->1->2
        while (p3 != null) {
            p2.next = p3.next;
            p3.next = p2;
            p1.next = p3;

            p1 = p2;
            if (p1.next == null) {
                break;
            }

            p2 = p1.next;
            p3 = p2.next;
        }

        return dummy.next;
    }
}

Incorrect Solutions

References