class Solution{
    public : 
        ListNode* mergeTwoLists(ListNode* l1, ListNode* l2){
            if(!l1 || !l2)
                return l1? l1: l2;
            if(l1->val > l2->val)
                swap(l1,l2);
            l1->next = mergeTwoLists(l1->next, l2);
            return l1;

        }
}

//At every step, pick the smaller head node, attach it to the result,and recursively merge the remaining lists
//TC : O(m+n)
//SC: O(m+n), recursion stack space