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

Answer 1

Here's another way of solving it which has O(n^2) time complexity and passes the LeetCode test. It counts the occurrences and then sorts (number, count) tuples so [-1, 0, 1, 2, -1, -4] becomes [(-4, 1), (-1, 2), (0, 1), (1, 1), (2, 1)]. Then it iterates from beginning picking first trying to pick each number twice and third greater if possible and add this to result. Then it picks number once and tries to find two greater numbers which sum to 0.

from collections import Counter

class Solution(object):
    def threeSum(self, nums):
        res = []
        counts = Counter(nums)
        num_counts = sorted(counts.items())

        # Handle the only case where we pick three same nums
        if counts[0] >= 3:
            res.append([0] * 3)

        for i, (first, first_count) in enumerate(num_counts):
            # Pick two of these and one greater
            if first_count >= 2 and first < 0 and -(first * 2) in counts:
                res.append([first, first, -(first * 2)])

            # Pick one and two greater
            for j in range(i + 1, len(num_counts)):
                second, second_count = num_counts[j]
                # Pick two of these as second and third num
                if second_count >= 2 and -first == 2 * second:
                    res.append([first, second, second])

                # Pick this as second num and third which is greater
                third = -(first + second)
                if third > second and third in counts:
                    res.append([first, second, third])

        return res