15_Three Sum

Level: medium

Tag: array

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.

Example:
Given nums = [-1, 0, 1, 2, -1, -4]
A solution set is:
[
  [-1, 0, 1],
  [-1, -1, 2]
]

Idea

• Sort first

• fix the smallest element, loop from left to right

• to find the second and third element, similar to Two Sum II - Input array is sorted _Q167

Time complexity

O(n^2)

space complexity

O(1)

Solution

class Solution(object):
    def threeSum(self, nums):
        """
        :type nums: List[int]
        :rtype: List[List[int]]
        """
        res = []
        nums.sort()
        for i in xrange(len(nums)-2):
            # to avoid duplicate for i
            if i > 0 and nums[i] == nums[i-1]:
                continue
            l, r = i+1, len(nums)-1
            while l < r:
                s = nums[i] + nums[l] + nums[r]
                if s < 0:
                    l +=1 
                elif s > 0:
                    r -= 1
                else:
                    res.append((nums[i], nums[l], nums[r]))
                    # to avoid duplicate for l
                    while l < r and nums[l] == nums[l+1]:
                        l += 1
                    # to avoid duplicate for r
                    while l < r and nums[r] == nums[r-1]:
                        r -= 1
                    l += 1; r -= 1
        return res