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18. 4sum

Array Two Pointers Sorting

Problem - 4sum

Medium

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.

 

Example 1:

Input: nums = [1,0,-1,0,-2,2], target = 0
Output: [[-2,-1,1,2],[-2,0,0,2],[-1,0,0,1]]

Example 2:

Input: nums = [2,2,2,2,2], target = 8
Output: [[2,2,2,2]]

 

Constraints:

  • 1 <= nums.length <= 200
  • -109 <= nums[i] <= 109
  • -109 <= target <= 109

Solutions

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class Solution:
    def fourSum(self, nums: List[int], target: int) -> List[List[int]]:
        nums.sort()
        n = len(nums)
        result = []

        for i in range(n):
            # no dupes
            if i > 0 and nums[i] == nums[i - 1]:
                continue

            for j in range(i + 1, n):
                if j > i + 1 and nums[j] == nums[j - 1]:
                    continue
                left, right = j + 1, n - 1
                while right > left:
                    total = nums[i] + nums[j] + nums[left] + nums[right]
                    if total == target:
                        result.append([nums[i], nums[j], nums[left], nums[right]])
                        right -= 1
                        left += 1

                        while right > left and nums[left] == nums[left - 1]:
                            left += 1
                        while right > left and nums[right] == nums[right + 1]:
                            right -= 1
                    elif total < target:
                        left += 1
                    else:
                        right -= 1
        return result

Submission Stats:

  • Runtime: 396 ms (47.36%)
  • Memory: 17.8 MB (55.34%)