Swap Nodes in Pairs

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

样例

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

/**
* Definition for singly-linked list.
* public class ListNode {
*     int val;
*     ListNode next;
*     ListNode(int x) { val = x; }
* }
*/
public class Solution {
/**
* @param head a ListNode
* @return a ListNode
*/
public ListNode swapPairs(ListNode head) {
// Write your code here
ListNode dummy = new ListNode(0);
dummy.next = head;

head = dummy;
while(head.next != null && head.next.next != null){
ListNode n1 = head.next, n2 = head.next.next;
//head->n1->n2...
//=>head->n2->n1...
head.next = n2;
n1.next = n2.next;
n2.next = n1;

head = n1;
}

return dummy.next;
}

}