LeetCode Problem : Swap Nodes in Pairs

Problem

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.

Code

ListNode *swapPairs(ListNode *head) {
    // Note: The Solution object is instantiated only once and is reused by each test case.
    if(!head)
        return 0;
    ListNode *first = head;
    ListNode *second = head->next;
    if(!second)
        return first;
    ListNode *third = second->next;
    second->next = first;
    first->next = swapPairs(third);
    return second;
}