Transform array to adjacency list
Last updated: July 14, 2025
Quick Overview
Given an array of edges representing a graph, transform it into an adjacency list representation. Each edge is defined by a pair of vertices, and the output should be a dictionary where each key is a vertex and its value is a list of adjacent vertices. Ensure that the adjacency list is constructed correctly to reflect the undirected nature of the graph.
PayPal
July 14, 2025149
15
664 solved
Given an array of edges representing a graph, transform it into an adjacency list representation. Each edge is defined by a pair of vertices, and the output should be a dictionary where each key is a vertex and its value is a list of adjacent vertices. Ensure that the adjacency list is constructed correctly to reflect the undirected nature of the graph.
Coding interviews at PayPal focus on problem-solving approach as much as the final solution. The interviewer wants to see you break down the problem, consider edge cases, and optimize iteratively. Communication throughout the process is key.
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 test this solution thoroughly?
- Can you optimize the space complexity of your solution?
- What if the input doesn't fit in memory?
- How would you modify your solution to handle streaming input?
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Problem Analysis
The problem requires transforming an array of edges into an adjacency list representation of an undirected graph. The adjacency list is inherently a mapping where each vertex points to a list of its a...
Approach
- Initialize a dictionary: Create an empty dictionary to store the adjacency list.
- Iterate through the edges: For each edge (u, v) in the input array:
- If vertex u is not in the dictio...
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