24. 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.

/**
* Definition for singly-linked list.
* public class ListNode {
*     int val;
*     ListNode next;
*     ListNode(int x) { val = x; }
* }
*/
public class Solution {
public ListNode swapPairs(ListNode head) {
if(head == null) return null;
ListNode fakehead = new ListNode(0);
fakehead.next = head;
ListNode p = fakehead;
ListNode q = p.next;
while(p.next != null && q.next != null){
p.next = q.next;
q.next = q.next.next;
p.next.next = q;
p = p.next.next;
q = q.next;

}
return fakehead.next;
}
}