3Sum

Question

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]
]

Tags

• Array
• Two Pointers

Thought

For each number in the array nums, run a two sum algorithm for all possible combinations. Remember to remove the duplicate combinations.

Code

class Solution(object):
    def threeSum(self, nums):
        """
        Here we use two pointers algorithm for two sum
        :type nums: List[int]
        :rtype: List[List[int]]
        """
        nums.sort()
        ans = []
        for i in xrange(len(nums) - 2):
            if i == 0 or nums[i] > nums[i - 1]:
                left = i + 1
                right = len(nums) - 1
                while left < right:
                    if nums[left] + nums[right] == -nums[i]:
                        ans.append([nums[i], nums[left], nums[right]])
                        left += 1
                        right -= 1
                        while left < right and nums[left] == nums[left - 1]:
                            left += 1
                        while left < right and nums[right] == nums[right + 1]:
                            right -= 1
                    elif nums[left] + nums[right] < -nums[i]:
                        left += 1
                        while left < right and nums[left] == nums[left - 1]:
                            left += 1
                    else:
                        right -= 1
                        while left < right and nums[right] == nums[right + 1]:
                            right -= 1
        return ans