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Which modification to the optimal flatten algorithm is necessary to handle this correctly?

hard🎤 Interviewer Follow-up Q15 of Q15
Tree: Depth-First Search - Flatten Binary Tree to Linked List
Suppose the problem is modified so that the binary tree nodes can be reused multiple times in the flattened linked list (i.e., nodes can appear multiple times). Which modification to the optimal flatten algorithm is necessary to handle this correctly?
ANo change needed; the current in-place reverse preorder traversal works as is.
BUse a preorder traversal to collect nodes in a list and rebuild the linked list allowing duplicates.
CModify the algorithm to clone nodes during traversal to allow multiple appearances.
DSwitch to a postorder traversal to ensure all duplicates are appended after processing children.
Step-by-Step Solution
Solution:

  1. Step 1: Understand reuse requirement

    Allowing nodes to appear multiple times means the original nodes cannot be simply rewired in-place without duplication.

  2. Step 2: Identify necessary modification

    Cloning nodes during traversal is required to create multiple instances, preserving original tree structure and allowing duplicates.

  3. Step 3: Why other options fail

    No change needed; the current in-place reverse preorder traversal works as is. fails because in-place rewiring destroys original nodes. Use a preorder traversal to collect nodes in a list and rebuild the linked list allowing duplicates. collects nodes but does not clone them. Switch to a postorder traversal to ensure all duplicates are appended after processing children. traversal order does not address duplication.

  4. Final Answer:

    Option C -> Option C

  5. Quick Check:

    Cloning nodes enables multiple appearances in flattened list [OK]
Quick Trick: Cloning nodes is needed for multiple appearances [OK]
Common Mistakes:
MISTAKES
  • Assuming in-place rewiring supports duplicates
  • Collecting nodes without cloning leads to lost references
  • Changing traversal order alone does not solve duplication
Trap Explanation:
PITFALL
  • Candidates often think traversal order or collecting nodes suffices, but cloning is essential for multiple reuse.
Interviewer Note:
CONTEXT
  • Tests candidate's ability to adapt algorithm to relaxed constraints and understand node ownership.
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