[Leetcode 24 链表]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.

加个表头节点，各种清晰。

class Solution {

public:
ListNode *swapPairs(ListNode *head) {
ListNode nodehead(0);
ListNode *pmove = &nodehead;
nodehead.next = head; //if have only one node,won't go into while
ListNode *first = head;
ListNode *second = head?head->next:NULL; //head may be NULL
while(first && second)
{
first->next = second->next;
second->next = first;
pmove->next = second;
pmove = first;
first = first?first->next:NULL;
second = first?first->next:NULL;
}
return nodehead.next;
}
};