I knew the two-pointer approach going in but spent way too long second-guessing the dedup logic.
Start by clarifying the problem constraints (e.g., array size, duplicates, expected time complexity). Then propose the optimal O(n^2) solution: sort the array and use a two-pointer technique for each fixed element, skipping duplicates to ensure uniqueness. Discuss trade-offs with brute force and hash-based approaches.
Pro tip: Mention that sorting enables efficient duplicate skipping and two-pointer search, and that the O(n^2) time complexity is optimal for this problem since the output can be O(n^2) in the worst case. Also, handle edge cases like arrays with fewer than 3 elements.
Ask about input size, duplicate handling, and expected time/space complexity. Confirm that triplets must be unique and indices cannot be reused.
Briefly describe brute force O(n^3), hash map O(n^2) with extra space, and the optimal sort + two-pointer O(n^2) with O(1) extra space (excluding output).
Explain: sort the array; for each index i, skip duplicates; use two pointers left=i+1 and right=n-1 to find pairs summing to -nums[i]; skip duplicates for left and right.
State time complexity O(n^2) due to nested loops, and space complexity O(1) extra (or O(n) if counting sorting space).
Walk through a small example like [-1,0,1,2,-1,-4] to demonstrate correctness and duplicate handling.
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