[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.

class Solution {
public:
    ListNode *swapPairs(ListNode *head) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        
        //check null list
        if(head==NULL) return head;
        //check single-node list;
        if(head->next==NULL) return head;
        
        ListNode *prev;
        ListNode *p1;
        ListNode *p2;
        p1=head;
        prev=head;
        do{
            p2=p1->next;
            p1->next=p2->next;
            p2->next=p1;
            if(prev!=head)
                prev->next=p2;
            else
                head=p2;
            prev=p1;
            p1=p1->next;
        }while(p1!=NULL && p1->next!=NULL);
        
        return head;
    }
};