15. 3Sum

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:

Given array nums = [-1, 0, 1, 2, -1, -4],

A solution set is:
[
  [-1, 0, 1],
  [-1, -1, 2]
]

本题应该是leetCode官网在近期添加了一个测试个例，网上好多的解决方法都是TLE。本人刚好找到一位博主分享，参考地址已在最后贴出。

class Solution:
    def threeSum(self, nums):
        """
        :type nums: List[int]
        :rtype: List[List[int]]
        """
        length, res = len(nums), []
        nums.sort()
        # length-2 的原因是3Sum是3个数相加， 下方变量left和right为最后两个
        # 数，所以没有必要令 i 循环最后两位数。
        for i in range(length - 2):
            if i > 0 and nums[i] == nums[i-1]:
                continue
            left, right = i + 1, length - 1
            while left < right:
                result = nums[i] + nums[left] + nums[right]
                if result == 0:
                    res.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 result < 0:
                    left += 1
                else:
                    right -= 1

        return res  

参考：https://blog.csdn.net/fuxuemingzhu/article/details/83115850