[leetcode][18]4Sum

Given an array S of n integers, are there elements a, b, c, and d in S such that a + b + c + d =
target? Find all unique quadruplets in the array which gives the sum of target.

Note: The solution set must not contain duplicate quadruplets.

For example, given array S = [1, 0, -1, 0, -2, 2], and target = 0.

A solution set is:
[
[-1,  0, 0, 1],
[-2, -1, 1, 2],
[-2,  0, 0, 2]
]

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import itertools
import collections
class Solution(object):
def fourSum(self, nums, target):
"""
:type nums: List[int]
:type target: int
:rtype: List[List[int]]
"""

ans = []
cache = collections.defaultdict(list)
for k,v in enumerate(nums):
cache[v].append(k)
explored = set()
for i in range(len(nums)):
for j in range(i+1,len(nums)):
for k in range(j+1,len(nums)):
remain = target -(nums[i]+nums[j]+nums[k])
if remain not in cache:
continue
else:
idxs = cache[remain]
if any(t in explored for t in itertools.permutations((nums[i],nums[j],nums[k],remain))):
continue
for idx in idxs:
if idx==i or idx == j or idx == k:
continue
ans.append([nums[i],nums[j],nums[k],remain])
[explored.add(t) for t in itertools.permutations((nums[i],nums[j],nums[k],remain))]
break
return ans

O(n*n*n)