Binary Tree Properties

·           The maximum number of nodes at level ‘l’ of a binary tree is 2^l.
Here level is number of nodes on path from root to the node (including root and node). Level of root is 0.
This can be proved by induction.
For root, l = 1, number of nodes = 2⁰= 1
Assume that maximum number of nodes on level l is 2^l
Since in Binary tree every node has at most 2 children, next level would have twice nodes, i.e. 2 * 2^l

·           Maximum number of nodes in a binary tree of height ‘h’ is 2^h – 1.
Here height of a tree is maximum number of nodes on root to leaf path. Height of a tree with single node is considered as 1.
This result can be derived from point 2 above. A tree has maximum nodes if all levels have maximum nodes. So maximum number of nodes in a binary tree of height h is 1 + 2 + 4 + .. + 2^h-1. This is a simple geometric series with h terms and sum of this series is 2^h – 1.
Some people consider, height of the root is considered as 0. In this convention, the above formula becomes 2^h+1 – 1

·           In a Binary Tree with N nodes, minimum possible height or minimum number of levels is Log₂(N+1)
If we consider the convention where height of a leaf node is considered as 0, then above formula for minimum possible height becomes  Log₂(N+1) ? – 1

·           A Binary Tree with L leaves has at least Log₂ L+ 1   levels
A Binary tree has maximum number of leaves (and minimum number of levels) when all levels are fully filled. Let all leaves be at level l, then below is true for number of leaves L.

L   <=  2^l-1 

            l =   Log₂L + 1

          where l is the minimum number of levels.

·           In Binary tree where every node has 0 or 2 children, number of leaf nodes is always one more than nodes with two children.

L = T + 1

Where L = Number of leaf nodes

           T  = Number of internal nodes with two children

·           If a binary tree has 0 or 2 children, then number of leaf nodes are always one more than nodes with two children.