Transform linked list to hash map
Last updated: November 5, 2025
Quick Overview
Given a linked list, transform it into a hash map where each key is the value of a node and the corresponding value is the number of times that value appears in the linked list. Your function should take the head of the linked list as input and return the resulting hash map.
Jane Street
November 5, 2025294
12
3,683 solved
Given a linked list, transform it into a hash map where each key is the value of a node and the corresponding value is the number of times that value appears in the linked list. Your function should take the head of the linked list as input and return the resulting hash map.
Jane Street uses this problem in the Phone Screen to evaluate your algorithmic thinking. They expect you to discuss multiple approaches, analyze trade-offs between them, and implement the optimal solution with clean, readable code.
What the Interviewer Expects
- Identify the correct data structure and algorithm for the problem
- Write clean, bug-free code with proper variable naming
- Analyze time and space complexity correctly
- Handle basic edge cases (empty input, single element)
- Communicate your thought process while coding
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
- Can you solve this iteratively instead of recursively (or vice versa)?
- How would you modify your solution to handle streaming input?
- How would you test this solution thoroughly?
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Problem Analysis
To solve the problem of transforming a linked list into a hash map, we can leverage the hash map (or dictionary) data structure to keep track of the frequency of each node's value. The reason a hash m...
Approach
- Initialize an empty hash map (dictionary in Python).
- Begin traversing the linked list from the head node.
- For each node, check if its value is already a key in the hash map.
- If it ...
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