Please provide answer in C++.

Define the internal path length, I(T), of a tree T, to be the sum of the depths of all the internal nodes in T. Likewise, define the external path length, E(T), of a tree T, to be the sum of the depths of all the external nodes in T. Show that if I is a binary tree with n internal nodes, then E(T) =I(T) + 2n. (Hint: Use the fact that we can build T from a single root node via a series of n expand External operations.)