292. 4Sum

Given an array nums of n integers, return an array of all the unique quadruplets [nums[a], nums[b], nums[c], nums[d]] such that:

• 0 <= a, b, c, d < n
• a, b, c, and d are distinct
• nums[a] + nums[b] + nums[c] + nums[d] == target

You may return the answer in any order.

Extend the 3Sum pattern to find all unique quadruplets that sum to a given target. This requires an additional layer of iteration over the two-pointer technique.

Requirements

1. Basic Functionality

• Find all unique quadruplets that sum to target
• No duplicate quadruplets in the result
• Handle edge cases (fewer than 4 elements, no valid quadruplets)

Example Usage

fourSum([1, 0, -1, 0, -2, 2], 0);
// [[-2, -1, 1, 2], [-2, 0, 0, 2], [-1, 0, 0, 1]]

fourSum([2, 2, 2, 2, 2], 8);
// [[2, 2, 2, 2]]

Real-World Context

• Portfolio optimization: Finding combinations of assets that meet a target allocation
• Network routing: Finding groups of nodes that satisfy bandwidth constraints
• Resource allocation: Distributing resources across teams to meet targets