class Solution{
    public: 
        ListNode* SwapNode(ListNode* head){
            if(!head || !head->next) return head;

            ListNode* dummy = new ListNode(0);
            dummy->next = head;

            ListNode* cur = head;
            ListNode* prev = dummy;

            while(cur && cur->next){
                ListNode* nextPair = cur->next->next;
                ListNode* second = cur->next;

                second->next = cur;
                cur->next = nextPair;
                prev->next = second;

                prev = cur;
                cur = nextPair;
            }
            ListNode* newHead = dummy->next;
            delete dummy;
            return newHead;
        }
};

//TC : O(n)
//SC : O(1)