My first instinct was to just iterate from the back and swap, but preserving relative order makes that messy.
Clarify the problem and constraints, then propose a two-pointer solution that processes the array from right to left, moving non-zero elements to the end and filling the remaining positions with zeros. Aim for O(n) time complexity and O(1) space, and discuss potential trade-offs or alternative approaches.
Pro tip: Mention that while the problem asks for zeros at the front, a common variant is moving zeros to the end; showing awareness of both demonstrates adaptability. Also, emphasize that in-place means no additional data structures, and constant space is crucial for large datasets.
Confirm that the array is mutable, that relative order of non-zero elements must be preserved, and that only constant extra space is allowed. Ask about edge cases like empty arrays or all zeros.
Describe a two-pointer approach: iterate from the end, maintain a write pointer for non-zero elements, and after processing, fill the beginning with zeros. This achieves O(n) time and O(1) space.
State that the algorithm runs in O(n) time because each element is visited once, and uses O(1) extra space since only a few variables are needed.
Mention that a naive approach with extra space would be O(n) space, which is not allowed. Also, note that if the order of non-zero elements didn't need to be preserved, a simpler swap-based approach could work.
Walk through a small example like [1,0,2,0,3] to show the step-by-step transformation to [0,0,1,2,3]. Test edge cases: empty array, all zeros, no zeros.
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