15. 3Sum.py

# Given an array nums of n integers, are there elements a, b, c in nums such that a + b + c = 0? Find all unique triplets in the array which gives the sum of zero.

# Note:

# The solution set must not contain duplicate triplets.

# Example:

# Given array nums = [-1, 0, 1, 2, -1, -4],

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

class Solution:
    def threeSum(self, nums: List[int]) -> List[List[int]]:
        nums, r = sorted(nums), set()
        for i in [i for i in range(len(nums)-2) if i < 1 or nums[i] > nums[i-1]]:
            d = {-nums[i]-n: j for j, n in enumerate(nums[i + 1:])}
            r.update([(nums[i], n, -nums[i]-n) for j, n in enumerate(nums[i+1:]) if n in d and d[n] > j])
        return list(map(list, r))