LeetCode Swap Nodes in Pairs

note the edge cases!

Easy!

/**
 * 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) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        
        if(head==NULL || head->next==NULL)
            return head;
        
        ListNode *p = head;
        ListNode *q = head->next;
        
        // first
        p->next = q->next;
        q->next = p;
        head = q;        
        
        // others
        while(p->next!=NULL && p->next->next!=NULL)
        {
            q = p->next->next;
            p->next->next = q->next;
            q->next = p->next;
            p->next = q;
            p = q->next;
        }
        
        return head;
    }
};