5. (17 points total) A full binary tree is a binary tree in which each interior node has exactly two children. Write a recursive Java method, boolean isFull(TreeNode root) that, given a reference to the root node of a binary tree, returns true if the binary tree is full. Assume a standard Binary TreeNode implementation with left child and right child references. (Note, since a single node or empty tree has no interior nodes, they should be considered full.)