[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.

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
struct ListNode* swapPairs(struct ListNode* head) {
    struct ListNode* p1;
    struct ListNode* p2;
    struct ListNode* tmp;
    if (!head || !(head->next)){
        return head;
    }
    
    p1 = head;
    p2 = p1->next;
    
    tmp = p2->next;
    head = p2;
    p2->next = p1;
    p1->next = swapPairs(tmp);
    
    return head;
}