15. 三数之和

https://leetcode.cn/problems/3sum/
中等
需要注意的点

1. nums.sort() 排序
2. if i > 0 and nums[i] == nums[i-1]: 这句判断

3. 跳过left right

   class Solution:
   def threeSum(self, nums: List[int]) -> List[List[int]]:
       if not nums or len(nums) < 3:
           return []
       size = len(nums)
       # 1
       nums.sort()
       res = []
       for i in range(size):
           if i > 0 and nums[i] == nums[i-1]:
               continue
           left = i+1
           right = size-1
           while left < right:
               sum = nums[i] + nums[left]+nums[right]
               if sum == 0:
                   res.append([nums[i], nums[left], nums[right]])
                   # 3 
                   while left < right and nums[left] == nums[left+1]:
                       left += 1
                   left += 1
                   while left < right and nums[right] == nums[right-1]:
                       right -= 1
                   right -= 1
               elif sum >0:
                   right -= 1
               else:
                   left += 1
       return res