Binary tree terminology in Data Structures Theory - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a binary tree?

A binary tree is a structure made of nodes where each node has at most two children called left and right child.

beginner

Define the term 'root' in a binary tree.

The root is the topmost node of a binary tree. It is the starting point from which all other nodes descend.

beginner

What is a 'leaf' node?

A leaf node is a node that has no children. It is at the bottom of the tree.

beginner

Explain the term 'parent' and 'child' in a binary tree.

A parent node is a node that has one or two children nodes. A child node is a node that descends from a parent node.

intermediate

What does 'subtree' mean in a binary tree?

A subtree is any node in the tree along with all its descendants. It forms a smaller tree inside the main tree.

In a binary tree, how many children can a node have at most?

A One

B Three

C Two

D Unlimited

By definition, each node in a binary tree can have at most two children.

What is the name of the node at the very top of a binary tree?

A Leaf

B Parent

C Child

D Root

The root is the topmost node from which all other nodes descend.

Which node has no children in a binary tree?

A Leaf

B Parent

C Root

D Subtree

Leaf nodes have no children and are at the bottom of the tree.

What do we call a node that has one or two nodes descending from it?

A Parent

B Leaf

C Child

D Root

A parent node has children nodes descending from it.

What is a subtree in a binary tree?

A A node with no children

B A node and all its descendants

C The entire tree

D Only the root node

A subtree includes a node and all nodes below it, forming a smaller tree.

Describe the main parts of a binary tree and their roles.

Think about the top node, nodes with children, nodes without children, and smaller trees inside.

Explain how the terms 'parent', 'child', and 'leaf' relate to each other in a binary tree.

Consider the family-like relationship between nodes.

Practice

1. In a binary tree, what do we call the topmost node that has no parent?

easy

A. Root

B. Leaf

C. Internal node

D. Child

Solution

1. Step 1: Understand the position of nodes in a binary tree

   The topmost node in a binary tree is the starting point and has no parent node above it.

2. Step 2: Identify the term for the topmost node

   This node is called the root because it is the base from which all other nodes branch out.

3. Final Answer:

   Root -> Option A

4. Quick Check:

   Top node = Root [OK]

Hint: Top node with no parent is always the root [OK]

Common Mistakes:

• Confusing root with leaf
• Thinking root has a parent
• Calling root a child

2. Which of the following correctly describes a leaf node in a binary tree?

easy

A. The topmost node

B. A node with exactly two children

C. A node with one child

D. A node with no children

Solution

1. Step 1: Recall the definition of a leaf node

   A leaf node is a node that does not have any children, meaning it is at the end of a branch.

2. Step 2: Match the definition with the options

   A node with no children states the node has no children, which matches the leaf node definition.

3. Final Answer:

   A node with no children -> Option D

4. Quick Check:

   Leaf node = no children [OK]

Hint: Leaf nodes have zero children, no branches below [OK]

Common Mistakes:

• Thinking leaf has children
• Confusing leaf with root
• Assuming leaf has one child

3. Consider this binary tree node description:
Node A has two children: Node B (left) and Node C (right). Node B has no children. Node C has one child: Node D (left).
Which of these nodes is an internal node?

medium

A. Node B only

B. Node D only

C. Node A and Node C

D. Node A only

Solution

1. Step 1: Define internal nodes

   Internal nodes have at least one child. Leaf nodes have none.

2. Step 2: Analyze each node's children

   Node A has two children (B and C), so it is internal. Node B has no children, so it is a leaf. Node C has one child (D), so it is internal. Node D has no children, so it is a leaf.

3. Final Answer:

   Node A and Node C -> Option C

4. Quick Check:

   Internal nodes = nodes with children [OK]

Hint: Internal nodes have one or two children, leaves have none [OK]

Common Mistakes:

• Calling leaf nodes internal
• Ignoring nodes with one child
• Confusing node labels

4. Identify the error in this statement about binary trees:
"A leaf node can have one child."

medium

A. Leaf nodes cannot have any children, so the statement is false.

B. Leaf nodes are always the root, so the statement is false.

C. Leaf nodes can have two children, so the statement is false.

D. Leaf nodes must have exactly one child, so the statement is true.

Solution

1. Step 1: Recall the definition of a leaf node

   A leaf node is defined as a node with no children at all.

2. Step 2: Evaluate the statement

   The statement says a leaf node can have one child, which contradicts the definition. Therefore, the statement is false.

3. Final Answer:

   Leaf nodes cannot have any children, so the statement is false. -> Option A

4. Quick Check:

   Leaf node = no children [OK]

Hint: Leaf nodes have zero children, never one [OK]

Common Mistakes:

• Thinking leaf can have children
• Confusing leaf with internal node
• Misunderstanding node roles

5. You have a binary tree where every internal node has exactly two children, and all leaves are at the same depth. What is this type of binary tree called?

hard

A. Complete binary tree

B. Perfect binary tree

C. Balanced binary tree

D. Full binary tree

Solution

1. Step 1: Understand the definitions of binary tree types

   A full binary tree has every node with 0 or 2 children. A complete binary tree is filled level by level left to right. A balanced binary tree has heights of subtrees differ by at most one. A perfect binary tree is full and all leaves are at the same depth.

2. Step 2: Match the given conditions

   The tree described has every internal node with exactly two children (full) and all leaves at the same depth, which matches the perfect binary tree definition.

3. Final Answer:

   Perfect binary tree -> Option B

4. Quick Check:

   Full + all leaves same depth = Perfect tree [OK]

Hint: Full + all leaves same depth = Perfect binary tree [OK]

Common Mistakes:

• Confusing complete with perfect
• Mixing balanced with perfect
• Ignoring leaf depth condition