[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.

利用三个指针p, pPre, pPrePre来操作，画个图就比较清晰了

/**
* 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) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
if (head == NULL)
return NULL;

ListNode *pPrePre = NULL;
ListNode *pPre = NULL;
ListNode *p = head;

while(p && p->next)
{
pPre = p;
p = p->next;

ListNode *pNext = p->next;

if (pPre == head)
head = p;

if (pPrePre)
pPrePre->next = p;

p->next = pPre;
pPre->next = pNext;

pPrePre = pPre;
p = pNext;
}

return head;
}
};