重命名：    src/algorithm/tree/binary/FetchAndRemoveRightMostNode.java -> src/algorithm/tree/binary/GeneralTreeOperation.java
	修改：      src/algorithm/tree/binary/avl/AVLTree.java
    add merge(AVLTree<E>), not finished
1.add private AvlNode<E> rotateIfNotBalance(AvlNode<E>target)
2.refactor rotate(),supporting balancing a tree that its left child and right child differ >=2