Leetcode: Three sum

Given a list of integers, add three integers at the same time so that the sum of those integers is 0. There should be no repetition of integers, for example [-1, 0, 1] is equivalent to [-1, 1, 0]. The solution makes use of the fact that duplicate numbers can be ignored. To do this, the numbers need to be sorted first. Then, if the next number is the same as the previous, it can be skipped. This does not affect the number of the other pointers, thus there can be the same number in different pointers. In total there are two loops needed. The outer loop iterates from the smallest to the third-largest number. For every iteration, a pointer p2 and p3 are defined. The pointer p2 starts directly one position right of p1 while p3 starts at the end of the list. We then need to distinguish three cases. If the sum is exactly 0, we can add the item to the results. If it is greater than 0, we move the pointer p3 to the left which makes the number smaller. If it is less than 0, we move p2 to the right which increases the total sum. We continue this approach until p2 and p3 meet, then continue with the outer iteration. Additionally, there is some logic to deal with duplicates.

from typing import List

class Solution:
  def threeSum(self, nums: List[int]) -> List[List[int]]:
    # Sort in place
    nums.sort()

    result = []
    for p1 in range(len(nums) - 2):
      if p1 > 0 and nums[p1] == nums[p1 - 1]:
        continue

      p2 = p1 + 1
      p3 = len(nums) - 1

      while p2 < p3:
        s = nums[p1] + nums[p2] + nums[p3]
        if s == 0:
          result += [[nums[p1], nums[p2], nums[p3]]]
          p3 -= 1

          while p2 < p3 and nums[p3] == nums[p3 + 1]:
            p3 -= 1
        elif s > 0:
          p3 -= 1
        else:
          p2 += 1

    return result


if __name__ == '__main__':
  s = Solution().threeSum
  print(s([-1,0,1,2,-1,-4]), [[-1,-1,2],[-1,0,1]])
  print(s([-4,-2,-2,-2,0,1,2,2,2,3,3,4,4,6,6]), [[-4, -2, 6], [-4, 0, 4], [-4, 1, 3], [-4, 2, 2], [-2, -2, 4], [-2, 0, 2]])
  print(s([-2,0,1,1,2]), [[-2,0,2],[-2,1,1]])