# Time:  O(n^2)
# Space: O(1)
#
# Given an array S of n integers,
# are there elements a, b, c in S such that a + b + c = 0?
# Find all unique triplets in the array which gives the sum of zero.
#
# Note:
# Elements in a triplet (a,b,c) must be in non-descending order. (ie, a <= b <= c)
# The solution set must not contain duplicate triplets.
#    For example, given array S = {-1 0 1 2 -1 -4},
#
#    A solution set is:
#    (-1, 0, 1)
#    (-1, -1, 2)

import collections

def threeSum(nums):
    """
        :type nums: List[int]
        :rtype: List[List[int]]
    """
    nums, result, i = sorted(nums), [], 0
    while i < len(nums) - 2:
        if i == 0 or nums[i] != nums[i - 1]:
            j = i + 1
            k = len(nums) - 1
            while j < k:
                if nums[i] + nums[j] + nums[k] < 0:
                    j += 1
                elif nums[i] + nums[j] + nums[k] > 0:
                    k -= 1
                else:
                    result.append([nums[i], nums[j], nums[k]])
                    j = j + 1
                    k = k - 1
                    while j < k and nums[j] == nums[j - 1]:
                        j += 1
                    while j < k and nums[k] == nums[k + 1]:
                        k -= 1
        i += 1
    return result

print(threeSum([-1, 0, 1, 2, -1, -4]))