Leetcode#24 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.

Difficulty: Medium

ListNode* swapPairs(ListNode* head) {
ListNode* first;
ListNode* second;
ListNode* third;
ListNode* temp = new ListNode(0);;
ListNode* beg;

if(!head || !head -> next) {
return head;
}
first = head;
beg = head->next;
while(first &&first->next)
{

third = first->next->next;
second = first->next;
temp ->next = second;
first->next = second->next;
second->next = first;
temp = first;
first = first->next;
}
return beg;
}