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Given the parent-pointer + ancestor set approach, if the tree is:

easy🧾 Trace Q3 of Q15
Tree: Depth-First Search - Lowest Common Ancestor of Binary Tree
Given the parent-pointer + ancestor set approach, if the tree is:
    3
   / \
  5   1
 / \ / \
6  2 0  8

and p=2, q=8, which node will be returned as their Lowest Common Ancestor?
ANode with value 3
BNode with value 5
CNode with value 1
DNode with value 2
Step-by-Step Solution
Solution:

  1. Step 1: Collect ancestors of p=2

    Ancestors are 2 -> 5 -> 3 -> None

  2. Step 2: Traverse ancestors of q=8

    Ancestors are 8 -> 1 -> 3 -> None

  3. Step 3: Find first common ancestor

    Common ancestor is 3

  4. Final Answer:

    Option A -> Option A

  5. Quick Check:

    3 is root and common ancestor [OK]
Quick Trick: Ancestor sets intersect at root 3 [OK]
Common Mistakes:
MISTAKES
  • Choosing node 5 or 1 assuming closer to p or q
  • Selecting node 2 or 8 which are not common ancestors
Trap Explanation:
PITFALL
  • Mistaking closest parent of one node as LCA instead of common ancestor
Interviewer Note:
CONTEXT
  • Tests ability to trace parent-pointer + ancestor set approach
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