Swap Nodes in Pairs

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

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

Your algorithm should use only constant space. You may not modify the values in the list, only nodes itself can be changed.

Solution

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode swapPairs(ListNode head) {

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

        ListNode prev = head;
        ListNode cur = head.next;
        ListNode next = cur.next;

        ListNode resultHead = cur;
        cur.next = prev;

        if(next.next == null){
            prev.next = next;
            return resultHead;
        }

        cur = next;
        next = next.next.next;

        while (next != null){
            prev.next = cur.next;
            prev.next.next = cur;

            prev = cur;
            cur = next;

            if(next.next == null){
                prev.next = cur;
                return resultHead;
            }

            else
                next = next.next.next;
        }

        prev.next = cur.next;
        cur.next.next = cur;
        cur.next = null;

        return resultHead;
    }
}