18.四数之和

跟三数之和完全一样的思路

class Solution:
    def fourSum(self, nums: List[int], target: int) -> List[List[int]]:
        indexs = {}
        for i, num in enumerate(nums):
            index = indexs.get(num, [])
            index.append(i)
            indexs[num] = index
        res = set()
        for i in range(len(nums)-3):
            for j in range(i+1, len(nums)-2):
                for l in range(j+1, len(nums)-1):
                    for k in indexs.get(target - nums[i] - nums[j] - nums[l], []):
                        if k not in [i,j, l]:
                            s = tuple(sorted([nums[i], nums[j], nums[l], target- nums[i] - nums[j] - nums[l]]))
                            res.add(s)
                            break
        return list(res)

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