(1) The binary tree is composed of three basic units, they are nodes, edges, and root.
(2) A complete binary tree of depth k shall have at least 2^k - 1 nodes, and at most 2^(k+1) - 1 nodes.
(3) If a binary tree has 20 leaf nodes and 30 nodes with only one child, the total number of nodes in the binary tree shall be 51.
3. Title_with_topic: Binary Trees and Huffman Codes