A tree in which all of the nodes can have two children or siblings (at most) is called a binary tree. A binary tree has the following characteristics:
- A tree contains, at most, 2l nodes at level l.
- If a binary tree contains m nodes at level l, it contains at most 2m nodes at level l+1.
- A tree contains 2d leaves and therefore 2d-1 non-leaf nodes, where d is its depth.
- A binary tree with n internal nodes has (n+1) external nodes.
- A binary tree with n nodes has exactly n+1 NULL links (see the following screenshot):