void mergeList(ListNode *a, ListNode *b, ListNode *&h, ListNode *&t) {

   h = t = NULL;

   while (a && b) {

     ListNode *c = NULL;

     if (a->val <= b->val) {

       c = a;

       a = a->next;

     }

     else {

       c = b;

       b = b->next;

     }

     if (!h)

       h = t = c;

     else {

       t->next = c;

       t = t->next;

     }

   }

   while (a) {

     t->next = a;

     t = t->next;

     a = a->next;

   }

   while (b) {

     t->next = b;

     t = t->next;

     b = b->next;

   }

 }

 ListNode *sortList(ListNode *head) {

   ListNode *prev = NULL;

   ListNode *h1 = NULL;

   ListNode *h2 = NULL;

   ListNode *t1 = NULL;

   ListNode *t2 = NULL;

   ListNode *node = NULL;

   int len = ;

   node = head;

   while (node && (++len))

     node = node->next;

   for (int l = ; l < len; l <<= ) {

     prev = NULL;

     h1 = NULL;

     h2 = NULL;

     t1 = NULL;

     t2 = NULL;

     node = head;

     while (node) {

       h1 = t1 = node;

       for (int i = ; node && i < l; i++) {

         t1 = node;

         node = node->next;

       }

       if (t1)

         t1->next = NULL;

       h2 = t2 = node;

       for (int i = ; node && i < l; i++) {

         t2 = node;

         node = node->next;

       }

       if (t2)

         t2->next = NULL;

       ListNode *h, *t;

       if (h2)

         mergeList(h1, h2, h, t);

       else {

         h = h1;

         t = t1;

       }

       if (!prev)

         head = h;

       else

         prev->next = h;

       t->next = node;

       prev = t;

     }

   }

   return head;

 }