There can be nodes with a single child. Otherwise an AVL tree would always have 2^n - 1 or 2^n - 2^(n-2) - 1 nodes. Imagine adding a node to a balanced tree wit

There can be nodes with a single child. Otherwise an AVL tree would always have 2^n - 1 or 2^n - 2^(n-2) - 1 nodes. Imagine adding a node to a balanced tree with 3 nodes.