Trees are recursive structures. Each node has children, and each child is the root of its own subtree. You will learn how to compute values by aggregating information from subtrees. Count nodes below you. Find maximum matchings. Calculate path sums.
All by processing children first, then combining their answers. By the end, you will solve tree problems that look impossible at first glance. You will know when to use post-order DFS, how to define state, and how to combine child values.
