LeetCode 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.

思路一：很直接的迭代方法，但要处理需要处理的情况细节。

代码如下：

/**
* Definition for singly-linked list.
* struct ListNode {
*     int val;
*     ListNode *next;
*     ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode* swapPairs(ListNode* head) {
ListNode* front,*back;
if(head == NULL || head->next == NULL)
return head;
front = head;back=head->next;head=back;
while(back->next != NULL && back->next->next != NULL)
{
ListNode* tmp = back->next;
front->next = back->next->next;
back->next = front;
back = front->next;
front = tmp;
}
front->next = back->next;
back->next = front;
return head;
}
};

思路二：也可以使用递归方法