[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.

/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
/*algorithm
1)three pointer, tail,shead,stail
2)swap shead,stail
3)move tail to shead
time O(n) space O(1)
*/
ListNode* swapPairs(ListNode* head) {
ListNode dummy(0),*tail = &dummy;
ListNode*shead,*stail;
dummy.next = head;
while(tail && tail->next&& tail->next->next){
shead= tail->next;
stail = shead->next;
shead->next = stail->next;
stail->next = shead;
tail->next = stail;
tail = shead;
}
return dummy.next;
}
};