Tree traversals

Three depth-first search methods

1. Pre-order: root, left, right
   
   Pre-order: F, B, A, D, C, E, G, I, H

2. In-order: left, root, right
   
   In-order: A, B, C, D, E, F, G, H, I

3. Post-order: left, right, root
   
   Post-order: A, C, E, D, B, H, I, G, F