I knew the properties going in but actually coding the rotations and recoloring under pressure is a different beast.
Start by clarifying requirements (e.g., language, operations needed, whether to include deletion) and then outline the red-black tree properties and invariants. Implement insertion with rotations and recoloring first, then deletion if time permits, and finally test with edge cases. Focus on clean, modular code and explain your reasoning as you go.
Pro tip: Emphasize the invariants and how each operation preserves them; interviewers care more about your understanding of why the tree stays balanced than about memorizing every rotation case. If stuck on deletion, offer to implement a simpler balanced tree (like AVL) or discuss the algorithm conceptually.
Ask about the expected operations (insert, delete, search), language preference, and whether to implement from scratch or use existing libraries. Confirm if deletion is required or if insertion-only is acceptable.
Explain the red-black tree properties: root is black, red nodes have black children, all paths from a node to leaves have the same number of black nodes, and leaves (NIL) are black. Define the node structure with color, key, value, left, right, and parent pointers.
Write the standard BST insert, color the new node red, then fix violations using rotations and recoloring. Cover all cases: uncle red (recolor), uncle black and node is inner/outer child (rotate and recolor).
Handle deletion by finding the node, replacing with successor/predecessor if needed, and then fixing double-black violations with rotations and recoloring. Explain the cases and how they restore invariants.
Walk through edge cases: inserting duplicate keys, deleting root, single node, and sequences that trigger each rotation case. Verify invariants after each operation, and discuss time complexity (O(log n) for all operations).
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.