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.

比较简单，指针操作。

/**
* 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 == NULL || head->next == NULL) {
return head;
}

ListNode *first = head;
ListNode *second = head->next;
ListNode *newhead = second;
ListNode *tmp = NULL;
ListNode *prev = NULL;
while (first != NULL && second != NULL) {
tmp = second->next;
second->next = first;
first->next = tmp;
if (prev != NULL) {
prev->next = second;
}

prev = first;
first = tmp;
if (tmp != NULL) {
second = tmp->next;
}
}

return newhead;
}
};