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.

Solution:

/**
* 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* lead = new ListNode(-1);
lead->next = head;
ListNode *p = lead, *tmp;
while(head && head->next)
{
tmp = head->next;
head->next = head->next->next;
tmp->next = head;
p->next = tmp;
head = head->next;
p = p->next->next;
}

return lead->next;
}
};