A binary tree T has n leaf nodes, the number of nodes of degree 2 in T is:

In an "m-ary" tree. the number of total nodes (N) is given by

N=mi + 1  ----(1)

Where,

i: Number of internal nodes

Also, in a tree, N=i + L   ----(2)

Where,

L=number of leaf nodes

Here m=2

From equation (1) and equation (2);

N = 2i + 1

2i + 1 = i + L

L = i + 1

The number of leaves are 1 plus the number of internal nodes in binary tree.

Here, given L=n, substitute above and we will get,

i = L - 1