Validate linked list complete
Last updated: December 25, 2025
Quick Overview
Given a singly linked list, implement a function to determine if the linked list is complete, meaning all nodes are filled and there are no gaps in the sequence of nodes. The function should return a boolean value: true if the linked list is complete, and false otherwise.
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December 25, 202529
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Given a singly linked list, implement a function to determine if the linked list is complete, meaning all nodes are filled and there are no gaps in the sequence of nodes. The function should return a boolean value: true if the linked list is complete, and false otherwise.
This coding problem is frequently asked during Technical Screen at Instacart. The interviewer is testing your ability to translate a problem into clean, working code while discussing time and space complexity. Instacart expects candidates to write production-quality code, not just solve the puzzle.
What the Interviewer Expects
- Recognize the underlying problem pattern (sliding window, two pointers, BFS/DFS, etc.)
- Discuss multiple approaches and trade-offs before coding
- Implement an optimal solution with clean, production-quality code
- Handle all edge cases including boundary conditions and invalid input
- Optimize both time and space complexity with clear justification
- Test your solution systematically with well-chosen examples
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
- Can you solve this iteratively instead of recursively (or vice versa)?
- What if the input doesn't fit in memory?
- What happens if the input contains duplicates?
- How would you parallelize this solution?
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Problem Analysis
To determine if a singly linked list is 'complete,' we need to check if all nodes in the list are filled sequentially without any gaps. This problem can be approached using a traversal technique, spec...
Approach
- Start from the head of the linked list.
- Use a pointer (or current node) to traverse through the linked list.
- Check at each step whether the current node is null.
- If you encounter a null b...
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