15.三数之和.py

#
# @lc app=leetcode.cn id=15 lang=python3
#
# [15] 三数之和
#
# https://leetcode-cn.com/problems/3sum/description/
#
# algorithms
# Medium (25.48%)
# Likes:    1797
# Dislikes: 0
# Total Accepted:    156.7K
# Total Submissions: 614.6K
# Testcase Example:  '[-1,0,1,2,-1,-4]'
#
# 给定一个包含 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
# ？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]，
# 
# 满足要求的三元组集合为：
# [
# ⁠ [-1, 0, 1],
# ⁠ [-1, -1, 2]
# ]
# 
# 
#

# @lc code=start
class Solution:
    def threeSum(self, nums: List[int]) -> List[List[int]]:
        # 排序 + 双指针 O(N^2) O(logN)
        n = len(nums)
        nums.sort()
        num_list = []
        if n < 3:
            return num_list
        for i in range(n-2):
            if nums[i] > 0:
                break
            if i > 0 and nums[i] == nums[i-1]:
                continue
            l, r = i+1, n-1
            while l < r:
                three_sum = nums[i] + nums[l] + nums[r]
                if three_sum < 0:
                    l += 1
                elif three_sum > 0:
                    r -= 1
                else:
                    num_list.append([nums[i], nums[l], nums[r]])
                    l += 1
                    r -= 1
                    while l < r and nums[l] == nums[l-1]:
                        l += 1
                    while l < r and nums[r] == nums[r+1]:
                        r -= 1

        return num_list
                

# @lc code=end