leetcode---swap-nodes-in-pairs---链表

Given a linked list, swap every two adjacent nodes and return its head.

For example,

Given1->2->3->4, you should return the list as2->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.

/**
* 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)
return NULL;

ListNode *p = head;
ListNode *pre = head;
bool first = true;
while(p && p->next)
{
ListNode *next = p->next;      // next = 2   ； next = 4
ListNode *next2 = next->next;  // next2 = 3  ； next2 = 5
next->next = p;                // 2->1       ； 4->3
p->next = next2;               // 1->4       ； 3->NULL

if(first)
{
head = next;
first = false;
}
else
pre->next = next;          //            ； 1->4
pre = p;                       // pre = 1    ； pre = 3
p = next2;                     // p = 3      ； p = NULL
}

return head;
}
};