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# 12.2. Binary Trees¶

## 12.2.1. Definitions and Properties¶

A binary tree is made up of a finite set of elements called nodes. This set either is empty or consists of a node called the root together with two binary trees, called the left and right subtrees, which are disjoint from each other and from the root. (Disjoint means that they have no nodes in common.) The roots of these subtrees are children of the root. There is an edge from a node to each of its children, and a node is said to be the parent of its children.

If $n_1, n_2, ..., n_k$ is a sequence of nodes in the tree such that $n_i$ is the parent of $n_i+1$ for $1 \leq i < k$, then this sequence is called a path from $n_1$ to $n_k$. The length of the path is $k-1$. If there is a path from node $R$ to node $M$, then $R$ is an ancestor of $M$, and $M$ is a descendant of $R$. Thus, all nodes in the tree are descendants of the root of the tree, while the root is the ancestor of all nodes. The depth of a node $M$ in the tree is the length of the path from the root of the tree to $M$. The height of a tree is the depth of the deepest node in the tree. All nodes of depth $d$ are at level $d$ in the tree. The root is the only node at level 0, and its depth is 0. A leaf node is any node that has two empty children. An internal node is any node that has at least one non-empty child.

Figure 12.2.1: A binary tree. Node $A$ is the root. Nodes $B$ and $C$ are $A$’s children. Nodes $B$ and $D$ together form a subtree. Node $B$ has two children: Its left child is the empty tree and its right child is $D$. Nodes $A$, $C$, and $E$ are ancestors of $G$. Nodes $D$, $E$, and $F$ make up level 2 of the tree; node $A$ is at level 0. The edges from $A$ to $C$ to $E$ to $G$ form a path of length 3. Nodes $D$, $G$, $H$, and $I$ are leaves. Nodes $A$, $B$, $C$, $E$, and $F$ are internal nodes. The depth of $I$ is 3. The height of this tree is 3.

Figure 12.2.2: Two different binary trees. (a) A binary tree whose root has a non-empty left child. (b) A binary tree whose root has a non-empty right child. (c) The binary tree of (a) with the missing right child made explicit. (d) The binary tree of (b) with the missing left child made explicit.

Figure 12.2.1 illustrates the various terms used to identify parts of a binary tree. Figure 12.2.2 illustrates an important point regarding the structure of binary trees. Because all binary tree nodes have two children (one or both of which might be empty), the two binary trees of Figure 12.2.2 are not the same.

Two restricted forms of binary tree are sufficiently important to warrant special names. Each node in a full binary tree is either (1) an internal node with exactly two non-empty children or (2) a leaf. A complete binary tree has a restricted shape obtained by starting at the root and filling the tree by levels from left to right. In the complete binary tree of height $d$, all levels except possibly level $d$ are completely full. The bottom level has its nodes filled in from the left side.

Figure 12.2.3: Examples of full and complete binary trees.

Figure 12.2.3 illustrates the differences between full and complete binary trees. 1 There is no particular relationship between these two tree shapes; that is, the tree of Figure 12.2.3 (a) is full but not complete while the tree of Figure 12.2.3 (b) is complete but not full. The heap data structure is an example of a complete binary tree. The Huffman coding tree is an example of a full binary tree.

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While these definitions for full and complete binary tree are the ones most commonly used, they are not universal. Because the common meaning of the words “full” and “complete” are quite similar, there is little that you can do to distinguish between them other than to memorize the definitions. Here is a memory aid that you might find useful: “Complete” is a wider word than “full”, and complete binary trees tend to be wider than full binary trees because each level of a complete binary tree is as wide as possible.