Exam-style question
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Describe what makes a rooted tree a binary tree. State the maximum number of children a node can have and explain whether a node must have two children.
Model answer
What a good answer should say
- A binary tree is a rooted tree in which each node has at most two children.
- Therefore, a node can have zero, one or two children, but it cannot have more than two.
- It is not necessary for every node to have two children.
Explanation
Why this works
The phrase 'at most two' includes zero and one as well as two. Confusing 'at most two' with 'exactly two' would incorrectly exclude valid binary trees.
Common mistake
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