18. 4Sum leetcode Python 2016 new Season

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:

• Elements in a quadruplet (a,b,c,d) must be in non-descending order. (ie, a ≤ b ≤ c ≤ d)
• 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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need a hash to store the value combinations. store the combinations, the rest part is the same as 3 sum. 

this method will take O(nn) time and O(n) space.

class Solution(object):
    def fourSum(self, nums, target):
        """
        :type nums: List[int]
        :type target: int
        :rtype: List[List[int]]
        """
        value_hash = {}
        nums.sort()
        result = []
        nums_length = len(nums)
        if nums_length < 4:
            return result
        for i in range(nums_length):
            for j in range(i + 1, nums_length):
                num_sum = nums[i] + nums[j]
                if num_sum not in value_hash:
                    value_hash[num_sum] = [[i, j]]
                else:
                    value_hash[num_sum].append([i, j])
        for i in range(nums_length):
            for j in range(i + 1, nums_length - 2):
                difference = target - nums[i] - nums[j]
                if difference in value_hash:
                    for part_sum in value_hash[difference]:
                        if part_sum[0] > j and [nums[i], nums[j], nums[part_sum[0]], nums[part_sum[1]]] not in result:
                            result.append([nums[i], nums[j], nums[part_sum[0]], nums[part_sum[1]]])
        return result