I knew the classic 3Sum variant but having a custom target T threw me off for a second because I kept second-guessing whether my duplicate-skipping logic still held.
Start by clarifying the problem constraints and edge cases, then explain the sorting and two-pointer strategy to achieve O(n^2) time. Walk through the algorithm step-by-step, emphasizing how to skip duplicates and maintain lexicographic order, and discuss space complexity and testing.
Pro tip: Mention that sorting enables both the two-pointer technique and natural lexicographic ordering, and that skipping duplicates at each level ensures uniqueness without extra space. Also, proactively discuss how to handle integer overflow when summing large values.
Ask about input size, possible duplicates, negative numbers, and whether the array can be modified. Confirm that output should be lexicographically sorted and that O(1) extra space excludes the output.
Explain that sorting the array first allows using two pointers to find pairs for each fixed element, reducing the problem to O(n^2). Emphasize that sorting also gives lexicographic order for free.
Describe iterating through the array, fixing each element, and using left/right pointers to find pairs summing to T minus the fixed element. Explain skipping duplicate elements at the outer loop and inner pointers to avoid duplicate triplets.
State that time complexity is O(n^2) due to nested loops, and space complexity is O(1) extra beyond the output (if sorting in-place). Mention that the output itself may take O(k) space where k is the number of triplets.
Cover cases like array length < 3, no solution, all zeros, large numbers causing overflow, and duplicate-heavy arrays. Suggest test cases to validate correctness and performance.
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