Web Reference: 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: a, b, c, and d are distinct. You may return the answer in any order. Example 1: Output: [[-2,-1,1,2],[-2,0,0,2],[-1,0,0,1]] Example 2: Output: [[2,2,2,2]] Constraints: any solutions. No comments yet. In-depth solution and explanation for LeetCode 18. 4Sum in Python, Java, C++ and more. Intuitions, example walk through, and complexity analysis. Better than official and forum solutions. To avoid duplicate quadruplets, we first sort the array and use a set to store unique results. While this guarantees correctness, checking every combination of four elements is extremely slow for larger inputs. Sort the array. Use four nested loops to iterate through all combinations of indices a < b < c < d.
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