public ListNode SortList(ListNode head)
{
if (head == null || head.next == null) return head;
ListNode result, left, right, ptr;
// find the one before mid
left = head;
ptr = findMid(head);
right = ptr.next;
// break the link
ptr.next = null;
// divide and conqure
left = SortList(left);
right = SortList(right);
// merge
if(left.val<right.val){
result = left;
left = left.next;
}else{
result = right;
right = right.next;
}
ptr = result;
ptr.next = null;
while(left!=null || right !=null){
if (right == null || (left!=null && left.val < right.val))
{
// pick from left
ptr.next = left;
left = left.next;
}
else
{
// pick from right
ptr.next = right;
right = right.next;
}
ptr = ptr.next;
ptr.next = null;
}
return result;
}
static void Main(string[] args)
{
ListNode head = new ListNode(5);
ListNode n1 = new ListNode(3);
ListNode n2 = new ListNode(1);
ListNode n3 = new ListNode(2);
ListNode n4 = new ListNode(4);
head.next = n1;
n1.next = n2;
n2.next = n3;
n3.next = n4;
Program p = new Program();
head = p.SortList(head);
}
private ListNode findMid(ListNode head)
{
ListNode ptr1 = head, ptr2 = head;
while (ptr2.next != null && ptr2.next.next != null)
{
ptr2 = ptr2.next.next;
ptr1 = ptr1.next;
}
return ptr1;
}