Find all quadruplets summing to target

## Problem Given an integer array `nums` (length `n`) and an integer `target`, return **all unique quadruplets** `[a,b,c,d]` such that: - `a + b + c + d == target` - The quadruplet uses **four distinct indices** from `nums` - The output contains **no duplicate quadruplets** (order of quadruplets and order within a quadruplet do not matter) ## Input / Output - **Input:** `nums: int[]`, `target: int` - **Output:** `List<List<int>>` of unique quadruplets ## Constraints (typical interview) - `0 <= n <= 2000` - Values may be negative and may repeat ## Example - `nums = [1,0,-1,0,-2,2]`, `target = 0` - Output could be: `[[-2,-1,1,2],[-2,0,0,2],[-1,0,0,1]]` ## Clarifications - If no quadruplet exists, return an empty list. - Duplicates in `nums` are allowed, but duplicates in the returned quadruplets are not.