The Department of Computer Science & Engineering

CSE 116
Introduction To Computer Science for Majors 2
Lecture B
Lecture Notes
Stuart C. Shapiro
Spring, 2003

Binary Trees

Riley Chapter 9

A binary tree is a tree in which every node has at most two children, which are distinguished: one is the left child; the other is the right child. If a node has only one child, that is either the left child or the right child.

Node 1 is the root.
Nodes 2, 3, and 5 are branch nodes.
Nodes 4, 6, and 7 are leaves.
Nodes 1 and 2 have two children.
Node 3 has only a right child.
Node 5 has only a left child.

Numerical Properties of Full Binary Trees
A full binary tree is one in which for any level d, either all the nodes at level d are leaves, or all the nodes at level d have two children.

If the root is at level 0:
each level, d contains 2d nodes;
a tree of height h contains 2h+1-1 nodes.
In binary:
level 0:1 =20nodes
level 1:10 =21nodes
level 2:100 =22nodes
level 3:1000 =23nodes
Total:1111 =24-1nodes

See for a recursive implementation of binary trees.

Traversal Algorithms
There are three standard algorithms for traversing (visiting the nodes of) binary trees: preorder; inorder; postorder. ("Pre", "in", and "post" refer to when the root is visited relative to the subtrees.)

  1. Preorder Traversal
    Visit the root, then the left subtree, then the right subtree.
  2. Inorder Traversal
    Visit the left subtree, then the root, then the right subtree.
  3. Postorder Traversal
    Visit the left subtree, then the right subtree, then the root.

Example: Expression Trees
An expression tree represents an expression by storing an operator in the root, and its operand expressions in the two subtrees.

Inorder: ((5 * (10 - 4)) / (18 / (2 * 3)))
Preorder: / * 5 - 10 4 / 18 * 2 3
Postorder: 5 10 4 - * 18 2 3 * / /
See the implementation of
Notice that preorder and postorder notations do not require parentheses.

Pseudocode evaluator of postfix expressions:

int postfixEvaluator(TokenStream stream) {
    Stack stack;
    Token token;
    while (stream.hasNext()) {
        token =;
        if (token instanceof int) stack.push(token);
        else { // token is an operator
            opnd2 = stack.pop();
            opnd1 = stack.pop();
            stack.push(apply(token, opnd1, opnd2));

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Copyright © 2003 by Stuart C. Shapiro. All rights reserved.
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Stuart C. Shapiro <>