My first instinct was to just nest four loops and call it a day, which obviously isn't going to fly at Google.
Start by clarifying the problem: whether the array can contain duplicates, if the output should be sorted, and if the quadruplets themselves need to be sorted. Then, propose a solution that sorts the array and uses a two-pointer technique within nested loops to find all unique quadruplets in O(n^3) time, which is optimal for this problem.
Pro tip: Mention that you can optimize by skipping duplicate elements and adding pruning conditions (e.g., if the smallest possible sum exceeds the target or the largest possible sum is less than the target) to reduce unnecessary iterations, showing attention to performance.
Ask about input constraints, duplicate handling, output format, and whether the quadruplets need to be sorted. This ensures you understand the problem fully before coding.
Sorting helps in efficiently skipping duplicates and using two pointers to find pairs that sum to a target.
Use two outer loops to fix the first two elements, then use two pointers (left and right) to find the remaining two elements that sum to the target minus the fixed sum.
After finding a valid quadruplet, skip duplicate elements for all four positions to ensure uniqueness in the result set.
State the time complexity O(n^3) and space complexity O(1) (excluding output). Discuss edge cases like empty array, insufficient elements, and large inputs.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.