Leetcode 15. 3Sum

https://leetcode.com/problems/3sum/

Medium

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]
]

---

• 先升序排序，然后遍历第一个元素，在其后面元素中，再用两个指针，一个从前往后，一个从后往前遍历。
• typing — Support for type hints — Python 3.7.4rc1 documentation
  • https://docs.python.org/3/library/typing.html?highlight=typing#module-typing
  • from typing import List

class Solution:
    def threeSum(self, nums: List[int]) -> List[List[int]]:
        res = []
        nums.sort()
        
        for i in range(len(nums) - 2):
            if i == 0 or nums[i] != nums[i - 1]:
                left, right = i + 1, len(nums) - 1
                
                while left < right:
                    sum = nums[i] + nums[left] + nums[right]
                
                    if sum < 0:
                        left += 1
                    elif sum > 0:
                        right -= 1
                    else:
                        # should be List of List
                        res.append( [ nums[i], nums[left], nums[right] ] )
                        
                        while left < right and nums[left] == nums[left + 1]:
                            left += 1
                            
                        while left < right and nums[right] == nums[right - 1]:
                            right -= 1
                            
                        # remember to move pointers
                        left += 1
                        right -= 1
        
        return res