The core idea isn't hard once you've done 3Sum before.
Start by clarifying constraints (array size, value ranges, duplicates) and expected output format. Then present a solution that sorts the array and uses a two-pointer technique within nested loops to efficiently find unique quadruplets, discussing time and space complexity. Optionally, mention alternative approaches like hashing and compare trade-offs.
Pro tip: Demonstrate awareness of duplicate handling by explaining how sorting and skipping repeated elements ensures uniqueness, and proactively discuss how you would scale the solution for large datasets, which is crucial at Snapchat.
Ask about input size, value ranges, whether the array can contain duplicates, and if the output should be sorted or in any order. Confirm the expected time/space complexity.
Mention the naive O(n^4) solution using four nested loops and explain why it's inefficient for large inputs, setting the stage for optimization.
Describe sorting the array, then using two nested loops for the first two numbers and a two-pointer technique for the remaining two, skipping duplicates to ensure unique quadruplets.
State the time complexity O(n^3) and space complexity O(1) or O(n) depending on sorting, and discuss edge cases like empty array, no solution, and multiple duplicates.
Briefly mention hashing-based solutions (e.g., using a hash map to store pair sums) and compare their trade-offs in terms of time, space, and implementation complexity.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.