# Avoid using set Python solution

• ``````class Solution(object):
def fourSum(self, nums, target):
"""
:type nums: List[int]
:type target: int
:rtype: List[List[int]]
"""
if len(nums) < 4:
return []
nums.sort()
dic = {}
for i in reversed(range(3, len(nums))):
if i == len(nums) - 1 or nums[i] != nums[i + 1]:
for j in reversed(range(2, i)):
if j == i - 1 or nums[j] != nums[j + 1]:
sum2 = nums[i] + nums[j]
dic[sum2] = dic.get(sum2, []) + [[j, i]]
result = []
for i in range(len(nums) - 3):
if i == 0 or nums[i - 1] != nums[i]:
for j in range(i + 1, len(nums) - 2):
if j == i + 1 or nums[j - 1] != nums[j]:
if target - nums[i] - nums[j] in dic:
for x in dic[target - nums[i] - nums[j]]:
if j < x[0]:
result.append([nums[i], nums[j], nums[x[0]], nums[x[1]]])
return result
``````

I first reversely found potential 3rd and 4th numbers and put them in dic in ascending order, and then found 1st and 2nd numbers and make sure 2nd < 3rd to avoid checking duplicates.

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