We use recursion extensively when working with binary trees because Recursion uses a stack which is like a tree. It's easier to tell left from right. Recursion allows us to backtrack back up the tree. The root is always equal to NULL. A complete binary tree is one in which Every layer is filled. The smaller data is on the left. Every layer is filled except the bottom one which is filled left to right. All pointers at the bottom have been assigned to NULL. The C++ reserved word that controls recursion is The word function. The word for The word while The word if