/* L: L0→L1→…→Ln-1→Ln to L0→Ln→L1→Ln-1→L2→Ln-2→..*/
/// <summary>
/// 1. empty
/// 2.only one element
/// 3.
/// </summary>
/// <param name="node"></param>
public linknode reorderlist(linknode head)
{
if (head == null || head.next == null)
{
return(head);
}
linknode start = null, res = null;
start = head;
res = start;
while (start != null)
{
linknode tail = start, prevtail = null;
while (tail.next != null)
{
prevtail = tail;
tail = tail.next;
}
if (start == tail || start.next == tail)
{
break;
}
linknode tx = start.next;
start.next = tail;
tail.next = tx;
prevtail.next = null;
start = tx;
}
return(res);
}
public void insert(int x)
{
linknode temp = new linknode(x);
if (head == null)
{
head = temp;
}
else
{
tail.next = temp;
}
tail = temp;
}
public linknode getmiddle(linknode head)
{
if (head == null || head.next == null)
{
return(head);
}
linknode slow = head, fast = head.next;
while (fast != null && fast.next != null)
{
fast = fast.next.next;
slow = slow.next;
}
return(slow);
}
/*
* follow same as merge sort.
* 1. divide the head into tow halves
*/
public linknode sortlinkedlist(linknode head)
{
if (head == null || head.next == null)
{
return(head);
}
linknode middle = getmiddle(head);
linknode nextmiddle = middle.next;
middle.next = null;
linknode left = sortlinkedlist(head);
linknode right = sortlinkedlist(nextmiddle);
linknode res = mergesortlinkedlist(left, right);
return(res);
}
public linknode mergesortlinkedlist(linknode leftstart, linknode righstart)
{
linknode res = null;
if (leftstart == null)
{
return(righstart);
}
if (righstart == null)
{
return(leftstart);
}
if (leftstart.item <= righstart.item)
{
res = leftstart;
res.next = mergesortlinkedlist(leftstart.next, righstart);
}
else
{
res = righstart;
righstart.next = mergesortlinkedlist(leftstart, righstart.next);
}
return(res);
}
public linknode(int x)
{
item = x;
next = null;
}