I knew the general shape of the answer but fumbled the empty sublist skipping logic on the first pass.
Start by defining a minimal Iterator interface with hasNext() and next() methods, then implement FlattenIterator using a stack of iterators to handle nested lists. Ensure hasNext() is idempotent and advances past empty sublists, while next() returns the next integer and throws NoSuchElementException when exhausted.
Pro tip: Emphasize that hasNext() must be idempotent and that the stack approach naturally gives O(depth) space; also mention that using an iterator over the outer list avoids index management and simplifies edge cases.
Specify a generic Iterator<T> interface with boolean hasNext() and T next() methods, and optionally a remove() method that throws UnsupportedOperationException.
Use a stack (Deque) to store iterators of lists at each level. Initialize by pushing an iterator over the outer list if it's not empty.
In hasNext(), while the stack is not empty, peek the top iterator; if it has a next element, check if it's a list or integer. If it's an empty list, pop and continue; if it's a non-empty list, push its iterator; if it's an integer, return true. If stack becomes empty, return false.
In next(), call hasNext() to ensure an element is available; if not, throw NoSuchElementException. Then pop the top iterator and return its next integer.
Explain that each element is pushed and popped once, giving O(1) amortized time per operation, and the stack depth is at most the nesting depth, giving O(depth) space. Discuss handling empty input, repeated hasNext() calls, and nested empty lists.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.
This part felt cleaner to me than the flatten problem.
Clarify the interface and requirements, then design a lazy iterator that advances to the next matching element only when needed, caching the result to avoid recomputation. Implement with careful state management to handle edge cases like empty iterators, repeated hasNext() calls, and null predicates, ensuring O(1) amortized time per element.
Pro tip: Mention that you would use a sentinel value or a boolean flag to track whether the next matching element has been computed, preventing redundant work and ensuring idempotent hasNext() calls. Also, discuss how you would test edge cases like an empty source iterator or a predicate that never matches.
Ask about the expected interface (e.g., Java's Iterator<Integer>), whether null elements are allowed, and the predicate's behavior. Confirm that hasNext() must be idempotent and that the iterator should be lazy.
Use a reference to the source iterator, the predicate, and a cached next element (or a flag indicating whether it's computed). Implement a private method to advance to the next matching element, updating the cache.
In hasNext(), if the cache is not computed, call the advance method; return whether a next element exists. In next(), call hasNext() to ensure the cache is ready, then return the cached element and mark it as not computed.
Ensure repeated hasNext() calls do not advance the source iterator. Handle empty source, null predicate (throw NPE), and source iterator throwing exceptions. Consider thread-safety if required.
Explain that each element is processed at most once, giving O(1) amortized time per element. Discuss test cases: empty source, no matches, all matches, alternating matches, and large inputs.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.
Start by briefly describing the two iterator implementations (e.g., array-based and linked-list-based) and their core operations. Then walk through unit tests for each, covering normal cases, edge cases, and error conditions. Finally, analyze the time and space complexity of each operation for both implementations, comparing trade-offs.
Pro tip: Emphasize that unit tests should not only verify correctness but also document expected behavior and catch regressions; mention that complexity analysis should consider amortized costs and worst-case scenarios, which is crucial for financial systems like Coinbase.
Briefly explain the two iterator types (e.g., array-based and linked-list-based) and their internal state (e.g., index vs. node pointer).
List key test cases: empty iterator, single element, multiple elements, hasNext/next behavior, and exceptions (e.g., NoSuchElementException).
For each test case, describe the setup, action, and expected outcome, highlighting differences between the two implementations.
For each operation (hasNext, next), state the time complexity for both implementations, noting any amortized or worst-case nuances.
Discuss the space overhead of each iterator, including auxiliary space and any additional data structures used.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.