Minimum Genetic Mutation
Given a start gene, an end gene, and a bank of valid genes, find the minimum number of mutations needed to change the start gene to the end gene, where each mutation changes only one character and results in a valid gene. Use BFS.
Why Interviewers Ask This
Google asks this question to evaluate a candidate's ability to model real-world biological processes as graph traversal problems. They specifically test proficiency in Breadth-First Search for finding shortest paths in unweighted graphs, attention to detail regarding edge constraints like single-character mutations, and the capacity to translate abstract logic into efficient code under pressure.
How to Answer This Question
Key Points to Cover
- Explicitly identifying the problem as a shortest path search requiring BFS
- Demonstrating understanding of graph modeling where nodes are genes and edges are valid mutations
- Using a Set or Hash Map for O(1) lookup efficiency during neighbor generation
- Implementing a visited mechanism to prevent infinite loops and redundant processing
- Correctly handling edge cases such as unreachable targets or invalid inputs
Sample Answer
Common Mistakes to Avoid
- Using Depth First Search (DFS) which finds a path but not necessarily the shortest one
- Failing to remove visited genes from the bank set, causing the algorithm to revisit nodes infinitely
- Generating invalid characters instead of strictly limiting changes to A, C, G, and T
- Not checking if the start gene equals the end gene initially, leading to unnecessary computation
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