LeetCode15.三数之和

问题描述：

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。

解决方法：

class Solution:
    def threeSum(self, nums: List[int]) -> List[List[int]]:
        
        hash_dict = {}
        for num in nums:
            if num in hash_dict:
                hash_dict[num] += 1
            else:
                hash_dict[num] = 1
        
        answer = []
        
        negative_num = sorted(i for i in hash_dict if i < 0)
        positive_num = sorted(i for i in hash_dict if i >= 0)
        
        if 0 in hash_dict and hash_dict[0] > 2: answer.append([0,0,0])
        
        for p in positive_num:
            for n in negative_num:
                s = -(n+p)
                if s in hash_dict:
                    if s in (n, p) and hash_dict[s] >1:
                        answer.append([n,p,s])
                    elif s < n:
                        answer.append([s,n,p])
                    elif p < s:
                        answer.append([n,p,s])
        
        return answer