/**
 * 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) {
        ListNode *h=head;
        if(head==NULL||head->next==NULL){
            return head;
        }
       // ListNode *h=head;
        stack<int> st;
        while(h){
            ListNode *temp=h;
            for(int i=0;i<2;++i){
                if(h){
                    st.push(h->val);
                }else{
                    return head;
                }
                h=h->next;
            }
            while(!st.empty()){
                temp->val=st.top();
                st.pop();
                temp=temp->next;
            }
            
        }
        
        return head;
        
    }
    
};