← Snowflake Interview Insights
I went straight to BFS with in-degree tracking which felt natural, but then they asked me to also sketch the DFS version and I fumbled explaining the three-color visited state.
Start by clarifying the problem and edge cases, then implement Kahn's algorithm (BFS) as the primary solution. Walk through both BFS and DFS approaches, emphasizing cycle detection and complexity. Finally, discuss trade-offs and extensions like streaming edges and multiple valid orderings.
Pro tip: Mention that Kahn's algorithm naturally detects cycles by checking if the processed count equals n, and that DFS uses recursion stack states. Also, note that the problem is equivalent to topological sorting, and Snowflake values scalable solutions for large DAGs.
Ask about input constraints (e.g., n size, duplicate edges, self-loops) and output format. Confirm that any valid ordering is acceptable and that an empty array indicates a cycle.
Build adjacency list and indegree array, enqueue nodes with indegree 0, then process queue while decrementing indegrees. If result length equals n, return ordering; else return empty array.
Use DFS with three states (unvisited, visiting, visited) to detect cycles. Perform post-order traversal to build topological order, reversing the result at the end.
Both approaches are O(V+E) time and O(V+E) space. BFS is iterative and easier to reason about for cycle detection; DFS can be more intuitive for some but risks stack overflow for large graphs.
For streaming edges, consider incremental topological sort or dynamic algorithms. For multiple valid orderings, note that any topological order is acceptable, but if a specific order is needed, use a priority queue (e.g., lexicographically smallest).
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.
Start by clarifying requirements and edge cases, then outline a two-phase solution: tokenize the input and evaluate using a precedence-aware parser (e.g., shunting-yard or recursive descent) that handles unary minus and right-associative exponentiation. Emphasize O(n) time and space, 64-bit integer arithmetic with truncation toward zero, and robust error handling for malformed input.
Pro tip: Mention that unary minus can be treated as a special operator with high precedence (but lower than exponentiation) and that you can avoid recursion depth issues by using an explicit stack; also note that division truncation toward zero differs from floor division, so use integer division carefully.
Ask about operator precedence, associativity, unary minus handling, integer overflow, division truncation, and error conditions. Confirm that spaces are ignored and that parentheses are balanced.
Tokenize the string into numbers, operators, and parentheses. Choose a parsing strategy: recursive descent with precedence climbing or shunting-yard with an output queue and operator stack.
Handle unary minus by converting it to a special token or by tracking context. Ensure exponentiation is right-associative and has highest precedence. Use 64-bit integers and truncate division toward zero.
Detect malformed input such as mismatched parentheses, invalid characters, consecutive operators, or division by zero. Raise clear errors.
Confirm O(n) time and space. Walk through examples including nested parentheses, unary minus, and mixed operators. Discuss potential overflow and how to handle it.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.