86. Partition List

分区列表，给定⼀个数 x，比x小的在左边，其余在右边，且相对位置不能发⽣变化。
由于相对位置不能发⽣变化，所以不能⽤类似冒泡排序的思想。

int[] arr = new[] { 1, 4, 2, 3, 6, 5, 2 };
SLinkedList<int> slist = new SLinkedList<int>();
slist.AppendRange(arr);
Console.WriteLine("Before: " + slist.Print());
var rslt = slist.PartionList(4);
Console.WriteLine(" After: " + rslt.Print());

/// <summary>
/// 分区列表，小的在左边，大的在右边，位置相对不变
/// </summary>
/// <typeparam name="T">结点类型</typeparam>
/// <param name="source">单链表</param>
/// <param name="pivot">支点</param>
/// <returns></returns>
public static SLinkedList<T> PartionList<T>(this SLinkedList<T> source, T pivot) where T: IComparable<T>
{
	// 构造2个新链表，遍历源链表，分别存储
	if (source.IsEmpty())
	{
		return new SLinkedList<T>();
	}
	if (source.Head == null || source.Head.Next == null)
	{
		return source;
	}
	var tmp = new SLinkedList<T>(source);
	var head = PartionList(tmp.Head, pivot);
	tmp.Clear();
	return new SLinkedList<T>(head);
}
private static SLinkedListNode<T> PartionList<T>(SLinkedListNode<T> head, T pivot) where T : IComparable<T>
{
	SLinkedListNode<T> before = new SLinkedListNode<T>();
	var beforeHead = before;
	SLinkedListNode<T> after = new SLinkedListNode<T>();
	var afterHead = after;
	while (head != null)
	{
		if (head.Data.CompareTo( pivot) < 0)
		{
			before.Next = head;
			before = before.Next;
		}
		else
		{
			after.Next = head;
			after = after.Next;
		}
		head = head.Next;
	}
	after.Next = null;
	before.Next = afterHead.Next;
	return beforeHead.Next;
}