Trees

Recursion is the natural fit; a stack/queue gives the iterative version.

Core syntax

• DFS orders — pre (node,L,R), in (L,node,R), post (L,R,node).
• BFS — a deque, processing level by level.

def inorder(node):
    if not node:
        return
    inorder(node.left)
    visit(node.val)
    inorder(node.right)

Watch out

• In-order of a BST yields sorted values.
• Base case first: if not node: return.