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Which approach correctly adapts preorder traversal to this scenario without extra space?

hard🎤 Interviewer Follow-up Q15 of Q15
Tree: Depth-First Search - Binary Tree Preorder Traversal
Suppose you want to perform a preorder traversal on a binary tree where nodes can have parent pointers but no left or right pointers. Which approach correctly adapts preorder traversal to this scenario without extra space?
AUse a modified iterative approach that tracks previously visited nodes to avoid revisiting
BUse recursion on parent pointers to simulate traversal
CIteratively traverse using parent pointers and a stack to track visited nodes
DUse Morris traversal by creating temporary threaded links on parent pointers
Step-by-Step Solution
Solution:

  1. Step 1: Understand traversal constraints

    Without left/right pointers, standard Morris or recursion is not applicable; parent pointers only allow upward traversal.

  2. Step 2: Identify correct approach

    Tracking previously visited nodes iteratively allows preorder traversal by moving up/down without extra space for recursion stack.

  3. Final Answer:

    Option A -> Option A

  4. Quick Check:

    Modified iterative approach handles parent-only trees without extra space [OK]
Quick Trick: Parent-only trees require tracking visited nodes iteratively [OK]
Common Mistakes:
MISTAKES
  • Trying to use recursion without child pointers
  • Attempting Morris traversal on parent pointers
  • Using stack without tracking visited nodes
Trap Explanation:
PITFALL
  • Naive recursion or Morris traversal fails without child pointers; tracking visited nodes is essential.
Interviewer Note:
CONTEXT
  • Tests candidate's ability to adapt traversal algorithms to non-standard tree representations.
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