[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;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *swapPairs(ListNode *head) {
        if (head == NULL || head->next == NULL) return head;
        
        ListNode **prev = &head;
        while ((*prev) != NULL && ((*prev)->next) != NULL) {
            ListNode *pcurr = *prev;
            ListNode *pnext = (*prev)->next;
            pcurr->next = pnext->next;
            pnext->next = pcurr;
            *prev = pnext;
            prev = &(pcurr->next);
        }
            
        return head;
    }
};