自适应哈夫曼编码

FGK：

//  leaves preceed internal nodes of the

//  same weight in a non-decreasing ranking

//  of weights using implicit node numbers:

叶子优先于 相同权重的内部节点 做权重非递减排序时 ，如权重相同 则以 节点编号作 判断依据。

//  1) leaves slide over internal nodes, internal nodes
//  swap over groups of leaves, leaves are swapped
//  into group leader position, but two internal
//  nodes never change positions relative
//  to one another.

----------------------