給你一個包含 n 個整數的陣列nums，判斷nums中是否存在三個元素 a，b，c ，使得a + b + c = 0 ？請你找出所有和為 0 且不重複的三元組。

注意：答案中不可以包含重複的三元組。

來源：力扣（LeetCode）
連結：https://leetcode-cn.com/problems/3sum

寫了一個效率極低的實現，備份以示警戒！

class Solution {

    struct Element
    {
        int value;
        int times;
        Element(int value, int times):value(value),times(times){};
    };
 
public:
    vector<vector<int>> threeSum(vector<int>& nums) {
        if(nums.size()<3) return {};
        vector<vector<int>> rs;
        Element zero(0, 0);         // zero
        vector<Element*> plusVec;   // plus
        vector<Element*> minusVec;  // minus
 

        map<int, Element*> plusMap;    // plus  value to Element map
        map<int, Element*> minusMap;   // minus value to Element map
        this->initElement(nums, zero, plusVec, minusVec, plusMap, minusMap);

        if(zero.times > 2) rs.push_back({0,0,0}); // {0, 0, 0}
        if(zero.times > 0
 
) // {0, minus, plus}
        {
            for(auto it=plusMap.begin(); it!=plusMap.end(); it++)
            {
                Element *element = it->second;
                if(minusMap.find(-element->value) == minusMap.end()) continue;
                rs.push_back({minusMap[-element->value]->value, 0, element->value});
            }
        }
        appendElement(rs, minusVec, plusMap); // {minus, minus, plus}
        appendElement(rs, plusVec, minusMap);  // {minus, plus, plus}
        return rs;
    }

    void initElement(vector<int>& nums, Element &zero, vector<Element*> &plusVec, vector<Element*> &minusVec, map<int, Element*> &plusMap, map<int, Element*> &minusMap)
    {
        for(int i=0; i<nums.size(); i++)
        {
            if(nums[i] == 0)
            {
                zero.times++;
                continue;
            }
            if(nums[i] > 0)
            {
                if(plusMap.find(nums[i]) == plusMap.end())
                {
                    Element *element = new Element(nums[i], 1);
                    plusMap[element->value] = element;
                    plusVec.push_back(element);
                }
                else (plusMap[nums[i]]->times)++;
                continue;
            }
            if(nums[i] < 0)
            {
                if(minusMap.find(nums[i]) == minusMap.end())
                {
                    Element *element = new Element(nums[i], 1);
                    minusMap[element->value] = element;
                    minusVec.push_back(element);
                }
                else (minusMap[nums[i]]->times)++;
            }
        }
    }

    void appendElement(vector<vector<int>> &rs, vector<Element*> &elementVec, map<int, Element*> &cmap)
    {
        for(int i=0; i<elementVec.size(); i++) // {minus, minus, plus}
        {
            Element *element = elementVec[i];
            if(element->times>1 && cmap.find(-element->value*2) != cmap.end())  
            {
                rs.push_back({element->value, element->value, cmap[-element->value*2]->value});
            }
            for(int j=i+1; j<elementVec.size(); j++)
            {
                Element *element2 = elementVec[j];
                if(cmap.find(-element->value-element2->value) != cmap.end())
                {
                    rs.push_back({element->value, element2->value, cmap[-element->value-element2->value]->value});
                }
            }
        }
    }
    
};