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.
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解答：
这道题目的话就是区分奇数个节点和偶数个节点，然后注意一下只有一个头节点、空链表的情况就好了。
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* struct ListNode *next;
* };
*/
struct ListNode* swapPairs(struct ListNode* head) {
struct ListNode *tmp, *new_head = head, *tail = NULL;
while(NULL != head&&NULL != head->next){
tmp = head->next;
head->next = head->next->next;
tmp->next = head;
if(head == new_head){
new_head = tmp;
}
else{
tail->next = tmp;
}
tail = head;
head = head->next;
}
if(NULL != head&&NULL != tail){
tail->next = head;
}
return new_head;
}