Which modification to the standard iterative approach is necessary to correctly reconstruct the tree?
hard🎤 Interviewer Follow-up Q15 of Q15
Tree: Depth-First Search - Construct Tree from Preorder and Inorder
Suppose the binary tree nodes can have duplicate values, and you are given preorder and inorder traversals with duplicates. Which modification to the standard iterative approach is necessary to correctly reconstruct the tree?
AUse a hash map from value to a list of all indices in inorder, and track which index to use next during construction.
BIgnore duplicates and build the tree as usual, since preorder and inorder uniquely identify the tree.
CSort the preorder array to remove duplicates before building the tree.
DUse a brute force approach with array slicing to handle duplicates correctly.
Step-by-Step Solution
Step 1: Understand the problem with duplicates
Duplicates cause ambiguity in mapping values to inorder indices, breaking the unique root index lookup.
Step 2: Modify data structure
Store all indices of each value in a list, and track which occurrence to use next to maintain correct subtree boundaries.
Step 3: Avoid naive approaches
Ignoring duplicates or sorting breaks traversal properties; brute force is inefficient and unnecessary with proper tracking.
Final Answer:
Option A -> Option A
Quick Check:
Tracking multiple indices per value resolves duplicate ambiguity [OK]
Quick Trick:Track all inorder indices per value to handle duplicates [OK]
Common Mistakes:
MISTAKES
Assuming unique values always
Sorting preorder breaks tree structure
Using brute force causes inefficiency
Trap Explanation:
PITFALL
Ignoring duplicates seems simpler but leads to incorrect tree reconstruction.
Interviewer Note:
CONTEXT
Tests candidate's ability to adapt standard algorithms to relaxed constraints.
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