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.

public class Solution{
public ListNode swapPairs(ListNode head) {
if (head == null || head.next == null)
return head;

ListNode second = head.next;
ListNode third = second.next;

second.next = head;
head.next = swapPairs(third);

return second;
}
}
//从题目的意思上看，就知道这题目有很多重复性的动作，可以通过递归来解决，不难~