struct ListNode 
{
    int val;
    ListNode *next;
    ListNode(int x) : val(x), next(NULL) {}
};


 ListNode *MergeTwoLists(ListNode *l1, ListNode *l2) 
 {
    if(!l1) return l2;
    if(!l2) return l1;

    ListNode* head = (l1->val < l2->val)? l1: l2;
    ListNode* p = head;
    head == l1? l1=l1->next: l2=l2->next;

    while(l1 && l2)
    {
        ListNode*& less = l1->val < l2->val? l1: l2;
        
        p->next = less;
        less = less->next;
        p = p->next;
    }

    if(l1) p->next = l1;
    if(l2) p->next = l2;

    return head;
}