void sortListHelper(ListNode* head, ListNode* tail) {
if (head -> next == tail) return;
/* Partition the list. */
ListNode* pre = head;
ListNode* cur = head -> next;
ListNode* pivot = cur;
while (cur -> next && cur -> next != tail) {
if (pivot -> val > cur -> next -> val) {
ListNode* temp = pre -> next;
pre -> next = cur -> next;
cur -> next = cur -> next -> next;
pre -> next -> next = temp;
}
else cur = cur -> next;
}
sortListHelper(head, pivot);
/* Here is the trick. */
while (pivot -> next != tail && pivot -> next -> val == pivot -> val)
pivot = pivot -> next;
if (pivot -> next != tail) sortListHelper(pivot, tail);
}
ListNode *sortList2(ListNode *head) {
ListNode *saved_head = head;
int count = 0,step;
int length = 1;
while ((head = head->next)) length++;
while (true) {
step = pow(2, count);
if (step >= length) break;
sortListHelper(&saved_head,step);
count++;
}
return saved_head;
}
ListNode* sortList(ListNode* head) {
ListNode* new_head = new ListNode(0);
new_head -> next = head;
sortListHelper(new_head, NULL);
return new_head -> next;
}
