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In an in-place flatten function for a binary tree, why might some nodes still have non-null left pointers after flattening?

medium🛠️🐞 Bug Fixing Q7 of Q15
Tree: Depth-First Search - Flatten Binary Tree to Linked List
In an in-place flatten function for a binary tree, why might some nodes still have non-null left pointers after flattening?
AThe tree is not a binary tree
BThe recursion does not visit all nodes
CThe algorithm fails to explicitly set left pointers to null after rewiring
DThe right pointers are not updated correctly
Step-by-Step Solution
Solution:

  1. Step 1: Understand flatten requirements

    All left pointers must be set to null in the flattened list.

  2. Step 2: Identify common bug

    Some implementations forget to nullify left pointers after rewiring right pointers.

  3. Step 3: Consequence

    Left pointers remain non-null, violating the singly linked list structure.

  4. Final Answer:

    Option C -> Option C

  5. Quick Check:

    Always nullify left pointers after rewiring [OK]
Quick Trick: Always set left pointers to null after rewiring [OK]
Common Mistakes:
MISTAKES
  • Assuming rewiring right pointers is sufficient
  • Missing base case in recursion
  • Confusing left and right pointer updates
Trap Explanation:
PITFALL
  • Forgetting to nullify left pointers looks like partial flattening
Interviewer Note:
CONTEXT
  • Tests attention to detail in pointer manipulation
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