Transform array to adjacency list
Last updated: October 24, 2025
Quick Overview
Given an array of pairs where each pair represents a directed edge between two nodes, transform the array into an adjacency list representation of a graph. The output should be a dictionary where each key is a node and the corresponding value is a list of nodes that it points to. Ensure that the solution handles cases with multiple edges and isolated nodes efficiently.
Twilio
October 24, 202514
8
4,615 solved
Given an array of pairs where each pair represents a directed edge between two nodes, transform the array into an adjacency list representation of a graph. The output should be a dictionary where each key is a node and the corresponding value is a list of nodes that it points to. Ensure that the solution handles cases with multiple edges and isolated nodes efficiently.
This coding problem is frequently asked during Onsite at Twilio. The interviewer is testing your ability to translate a problem into clean, working code while discussing time and space complexity. Twilio expects candidates to write production-quality code, not just solve the puzzle.
What the Interviewer Expects
- Quickly identify the optimal approach and its theoretical basis
- Handle complex algorithm design with multiple interacting components
- Write concise, elegant code under time pressure
- Prove correctness of your approach and discuss alternative solutions
- Optimize beyond the obvious: discuss constant factor improvements
- Address follow-up variations and explain how the solution generalizes
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
- How would you parallelize this solution?
- Can you solve this in a single pass?
- What if the input doesn't fit in memory?
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Problem Analysis
The problem requires transforming an array of directed edges into an adjacency list representation of a graph. This is a classic graph representation problem where we utilize a dictionary (or hashmap)...
Approach
- Initialize an empty dictionary to hold the adjacency list.
- Iterate through each pair in the input array:
- For each edge (a, b), check if 'a' is already a key in the dictionary. If not, initi...
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