What is the max depth of a node? It is (for itself) plus the max depth of its tallest child.
For a leaf, the max depth is (itself). For an empty tree, the depth is .
This is a recursive definition. The base case is when the node is null (depth ). The recursive case combines the depths of left and right children.
This pattern applies to many tree problems. Define the answer for a node in terms of the answers for its children.