A postorder tree traversal.
Depth-first traversal (dotted path) of a binary tree:
Pre-order (node visited at position red ●): F, B, A, D, C, E, G, I, H;
In-order (node visited at position green ●): A, B, C, D, E, F, G, H, I;
Post-order (node visited at position blue ●): A, C, E, D, B, H, I, G, F.