15-3sum.py

"""
Given an array nums of n integers, are there elements a, b, c in nums 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.

Example:

Input: [-1, 0, 1, 2, -1, -4],
Output:
[
  [-1, 0, 1],
  [-1, -1, 2]
]
"""


class Solution:
    def threeSum(self, nums):
        """
        :type nums: List[int]
        :rtype: List[List[int]]
        """
        res = []
        nums.sort()
        n = len(nums)
        added = set()
        for i in range(n - 2):
            if i > 0 and nums[i] == nums[i - 1]:
                continue
            l, r = i + 1, n - 1
            while l < r:
                s = nums[i] + nums[l] + nums[r]
                if s < 0:   # it should choose large number
                    l += 1
                elif s > 0:  # it should choose small number
                    r -= 1
                else:
                    ans = [nums[i], nums[l], nums[r]]
                    key = ''.join(map(str, ans))
                    if key not in added:
                        res.append(ans)
                        added.add(key)
                    l += 1
                    r -= 1
        return res


if __name__ == "__main__":
    nums = [-1, 0, 1, 2, -1, -4]
    print(Solution().threeSum(nums))