19 After converting a tree into a binary tree, the shape of the binary tree is ( ).
A. Only
B. There are many
C. There are many, but none of the root nodes have left children
D. There are many, but none of the root nodes have right children
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20 Which of the following statements is correct ().
A. There are at most two subtrees in a binary tree, and there are left and right subtrees
B. A binary tree is an ordered tree of degree 2
C. There is no left-right distinction when the node has only one child in a binary tree.
D. There must be a node of degree 2 in the binary tree
21 There are altogether ( ) binary trees with 7 nodes, and they are not similar to each other.
A. 429
B. 428
C. 427
D. 426
22 In the Huffman tree, if the encoding length is only allowed to be less than or equal to 4, in addition to the determined encodings of two characters as 0 and 10, at most ( ) characters can be encoded.
A. 4
B. 3
C. 5
D. 6
23 In the preorder, inorder and postorder sequences of the binary tree, the order of all leaf nodes is( ).
A. Not the same
B. Exactly the same
C. the same in preorder and inorder sequence, but different from that in postorder sequence
D. the same in inorder and postorder sequence, but different from that in perorder sequence
E. the same in inorder and postorder sequence, but different from that in perorder sequence
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