Transform linked list to hash map
Last updated: January 29, 2026
Quick Overview
Given a singly linked list, transform it into a hash map where each key is the value of a node and the corresponding value is the node's index in the list. The output should be a hash map that accurately reflects the mapping of node values to their respective positions. Ensure that the solution handles cases with duplicate values appropriately by storing the last index of each value.
Citadel
January 29, 20265
8
233 solved
Given a singly linked list, transform it into a hash map where each key is the value of a node and the corresponding value is the node's index in the list. The output should be a hash map that accurately reflects the mapping of node values to their respective positions. Ensure that the solution handles cases with duplicate values appropriately by storing the last index of each value.
This coding problem is frequently asked during Take-home Project at Citadel. The interviewer is testing your ability to translate a problem into clean, working code while discussing time and space complexity. Citadel expects candidates to write production-quality code, not just solve the puzzle.
What the Interviewer Expects
- Recognize the underlying problem pattern (sliding window, two pointers, BFS/DFS, etc.)
- Discuss multiple approaches and trade-offs before coding
- Implement an optimal solution with clean, production-quality code
- Handle all edge cases including boundary conditions and invalid input
- Optimize both time and space complexity with clear justification
- Test your solution systematically with well-chosen examples
Key Topics to Cover
How to Approach This
- Clarify input constraints and edge cases before writing code.
- Walk through your approach verbally and confirm with the interviewer before coding.
- Start with a brute force solution, then optimize. Mention time and space complexity.
- Test your solution with examples, including edge cases like empty input or duplicates.
- Consider common patterns: sliding window, two pointers, hash map, BFS/DFS, dynamic programming.
Possible Follow-up Questions
- What if the input doesn't fit in memory?
- Can you solve this iteratively instead of recursively (or vice versa)?
- What happens if the input contains duplicates?
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Problem Analysis
The problem requires us to traverse a singly linked list and construct a hash map from it. The key pattern here is linear traversal of the linked list, as we need to access each node exactly once to r...
Approach
- Initialize an empty hash map (dictionary in Python) to store the mapping of node values to their indices.
- Start traversing the linked list from the head node.
- For each node, check its value a...
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