Remove Nth Node From End of List
Given the head of a linked list, remove the $n$-th node from the end of the list and return its head. Use the two-pointer technique.
Why Interviewers Ask This
Oracle interviewers use this problem to assess a candidate's ability to optimize space complexity while manipulating linked structures. They specifically evaluate whether you can implement the two-pointer technique efficiently without allocating extra memory for arrays or stacks, demonstrating strong algorithmic thinking and mastery of pointer arithmetic in constrained environments.
How to Answer This Question
1. Clarify edge cases immediately: Ask if n is guaranteed to be valid, if the list can be empty, or if n equals the list length. Oracle values candidates who validate inputs before coding.
2. Propose the Two-Pointer Strategy: Explain that instead of calculating total length first (requiring two passes), you will maintain a gap of n nodes between two pointers to achieve the goal in a single pass.
3. Define Pointer Roles: Assign the 'fast' pointer to move n steps ahead first. Then, move both 'slow' and 'fast' pointers one step at a time until 'fast' reaches the end.
4. Handle Removal Logic: Detail how 'slow' will point to the node just before the target. Use a dummy head node to simplify removal logic, especially when deleting the first node.
5. Refine Code Structure: Write clean code handling the dummy node setup, the initial fast movement loop, the synchronized traversal, and the final pointer reassignment.
Key Points to Cover
- Demonstrating understanding of O(1) space complexity constraints
- Using a dummy node to eliminate special case logic for head removal
- Explaining the maintenance of a fixed gap between two pointers
- Validating input parameters before implementing the algorithm
- Executing a single-pass traversal for optimal efficiency
Sample Answer
To solve the 'Remove Nth Node From End of List' problem efficiently, I would prioritize a single-pass solution using the two-pointer technique, which aligns with Oracle's focus on performance optimization. First, I'd add…
Common Mistakes to Avoid
- Failing to use a dummy node, leading to complex conditional checks for head deletion
- Calculating total list length first, resulting in inefficient two-pass solutions
- Incorrectly initializing the fast pointer, causing off-by-one errors in the gap calculation
- Not handling the edge case where n equals the list length (removing the head)
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