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;
* struct ListNode *next;
* };
*/
struct ListNode* swapPairs(struct ListNode* head) {
if (head == NULL) return head;

struct ListNode* curr = head;
struct ListNode* nt = curr->next;
if (nt == NULL) return head;
struct ListNode* nh = (struct LsitNode*) malloc(sizeof(struct ListNode));

struct ListNode* prev = nh;
while(nt != NULL) {
struct ListNode* tmp = nt->next;
nt->next = curr;
curr->next = tmp;
prev->next = nt;
prev = curr;
curr = curr->next;
if (curr != NULL) nt = curr->next;
else break;
}

return nh->next;

}