LeetCode 15 :三数之和（3Sum）

╰+攻爆jí腚メ 2023-07-17 04:36 16阅读 0赞

## 英文题目 ##

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 nums = [2, 7, 11, 15], target = 9,
    
    Because nums[0] + nums[1] = 2 + 7 = 9,
    return [0, 1].

## 中文题目 ##

给你一个包含 n 个整数的数组 nums，判断 nums 中是否存在三个元素 a，b，c ，使得 a + b + c = 0 ？请你找出所有满足条件且不重复的三元组。

注意：答案中不可以包含重复的三元组。

**示例:**

给定数组 nums = [-1, 0, 1, 2, -1, -4]，
    Given array nums = [-1, 0, 1, 2, -1, -4],
    
    A solution set is:
    [
      [-1, 0, 1],
      [-1, -1, 2]
    ]

## 解答 ##

#### Python ####

class Solution:
        # 思路: 先排序, 然后规定第一个数不动, 后面双指针法求和.并且去重
        def threeSum(self, nums: 'List[int]') -> 'List[List[int]]':
            n = len(nums)
            res = []
            if n < 3:
                return res
            nums.sort()
    
            for i in range(n):
                if i > 0 and nums[i] == nums[i - 1]:  # 判断固定的数是否重复
                    continue
                left = i + 1
                right = n - 1
                while left < right:
                    cur = nums[i] + nums[left] + nums[right]
                    if cur == 0:
                        res.append([nums[i], nums[left], nums[right]])
                        while left < right and nums[left] == nums[left + 1]:  # 判断是否重复
                            left += 1
                        while right < left and nums[right] == nums[right - 1]:
                            right -= 1
                        left += 1
                        right -= 1
                    elif cur > 0:
                        right -= 1
                    else:
                        left += 1
            return res