leetcode15 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]
]
"""
"""
two Sum用了hash。而Three Sum不需要用
先对数组排序
然后用三个指针 i, l, r 来进行遍历
先确定一个i的值，将i指向第一个不重复的值，l 指向头，r指向尾
第二层循环 while l<r
如果和大于0， r左移
如果和小于0， l右移
如果和等于0，保存结果
将l，r分别移到不重复这个值的位置
l，r再同时左移和右移
"""
class Solution:
    def threeSum(self, nums):
        res = []
        nums.sort()
        for i in range(len(nums) -2):
            if i > 0 and nums[i] == nums[i-1]:
                continue
            l, r = i+1, len(nums)-1
            while l < r:
                if nums[i] + nums[l] + nums[r] > 0:
                    r -= 1
                elif nums[i] + nums[l] + nums[r] < 0:
                    l += 1
                else:
                    res.append([nums[i], nums[l], nums[r]])
                    while l < r and nums[l] == nums[l+1]:
                        l += 1
                    while l < r and nums[r] == nums[r-1]:
                        r -= 1
                    l += 1
                    r -= 1
        return res