LeetCode 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.

交换两个结点，很简单，别忘记交换完的结点与前面结点的连接。

/**
* 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) {
if(!head || !head->next) return head;
ListNode* node1;
ListNode* node2;
ListNode* node3;
node1 = head->next;
node2 = node1->next;
node1->next = head;
head->next = node2;
head = node2;
ListNode* realhead = node1;
ListNode* pre = node1->next;
while(head && head->next){
node1 = head->next;
node2 = node1->next;
node1->next = head;
head->next = node2;
head = node2;
pre->next = node1;
pre = node1->next;
}
return realhead;
}
};