4. (2 points) A rooted tree has a special noder called the root and zero or more rooted subtrees T, ..., Tk each of whose roots are connected to r. The root of each subtree is called the child of r and r is called the parent of each child. Nodes with no children are called leaf nodes. A binary tree is a rooted tree in which each node has at most two children. Show by induction that in any binary tree the number of nodes with two children is exactly one less than the number of leaves