python - Three sum algorithm solution

Question

Original Problem Statement:

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: 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]]

---

I solved the Two Sum problem on LeetCode some time back and I thought of using it to solve the three sum as well. My idea is that for every element find two elements in the remaining list which sum up to element * -1 to get 0. However, this code doesn't pass all the test, for example

Input: [-4,-2,-2,-2,0,1,2,2,2,3,3,4,4,6,6]
Output: [[-4,-2,6],[-4,0,4],[-4,1,3],[-4,2,2]]
Expected: [[-4,-2,6],[-4,0,4],[-4,1,3],[-4,2,2],[-2,-2,4],[-2,0,2]]

I don't really know what's wrong. Can someone be kind enough to explain the problem to me.

class Solution(object):
    def threeSum(self, nums):
        """
        :type nums: List[int]
        :rtype: List[List[int]]
        """
        def twoSum(self, nums, target):
            targ = target
            for index, i in enumerate(nums):
                targ -= i
                if targ in nums[index+1:]:
                    return [nums[index], nums[nums[index+1:].index(targ)+index+1]]
                else:
                    targ = target
            return None
        res = []
        for index, i in enumerate(nums):
            target = i * -1
            num = nums[:index] + nums [index+1:]
            ans = twoSum(self, num, target)
            if ans != None:
                temp = ans + [i]
                temp.sort()
                res.append(temp)
        print(res)
        import itertools
        res.sort()
        res = list(res for res,_ in itertools.groupby(res))
        return res

Original Question: https://leetcode.com/problems/3sum/description/

The one I'm using to solve this: https://leetcode.com/problems/two-sum/description/

Answer 1

using itertools.

import itertools
stuff = [-1, 0, 1, 2, -1, -4]
stuff.sort()
ls = []
for subset in itertools.combinations(stuff, 3):
    if sum(list(subset))==0:
        # first I have sorted the list because of grouping
        # Ex: [-1, 0, 1] and [0, 1, -1] are build with the same element
        # so here is avoiding this.
        if list(subset) not in ls:
            ls.append(list(subset))
print(ls)

input/output

input : [-1, 0, 1, 2, -1, -4]
output : [[-1, -1, 2], [-1, 0, 1]]
input : [-4,-2,-2,-2,0,1,2,2,2,3,3,4,4,6,6] 
output: [[-4, -2, 6], [-4, 0, 4], [-4, 1, 3], [-4, 2, 2], [-2, -2, 4], [-2, 0, 2]]