地图不以递归方式存储值

Question

我不知道为什么，但是当递归尝试添加值然后返回它时，函数返回后的映射删除了所有元素，我不明白这是什么问题。

static int SumNum(string target, string m[], map<string, int> memo = {}){

        if (memo.find(target) != memo.end()) {
            cout << memo.find(target)->second<<"\n";
            return memo.find(target)->second;
        }
        if (target == "") return 1;

        int totalCount = 0;
        
    for(auto i = 0;i < 10; i++)
    {   
        if (target.find(m[i]) == 0){
            
            int numOfWays = SumNum(target.substr(m[i].length()),m,memo);
            totalCount += numOfWays;
            
        }
    }
    memo.emplace(target, totalCount);
    return totalCount;
}
int main(){
    string ar[10] = {"e","ee","eee","eeee","eeeee","eeeeee","f","fdc","d","c"};
    string target = "eeeeeeeeefdc";
    auto answer = SumNum(target,ar);
    cout << answer;
}

Answer 1

我认为@Max 的意思是：

static int SumNum(std::string target, std::string m[], std::map<std::string, int>& memo /* pass by reference */){

        if (memo.find(target) != memo.end()) {
            std::cout << memo.find(target)->second<<"\n";
            return memo.find(target)->second;
        }
        if (target == "") return 1;

        int totalCount = 0;
        
    for(auto i = 0;i < 10; i++)
    {   
        if (target.find(m[i]) == 0){
            
            int numOfWays = SumNum(target.substr(m[i].length()),m,memo);
            totalCount += numOfWays;
            
        }
    }
    memo.emplace(target, totalCount);
    return totalCount;
}
int main(){
    std::map <std::string, int> map;
    std::string ar[10] = {"e","ee","eee","eeee","eeeee","eeeeee","f","fdc","d","c"};
    std::string target = "eeeeeeeeefdc";
    auto answer = SumNum(target, ar, map);
    std::cout << answer;
}

Answer 2

感谢@Max，我确实像你说的那样

static int SumNum(string target, string m[], map<string, int> *memo = new map<string, int>()) {
        
        if (memo->find(target) != memo->end()) {
            cout << memo->find(target)->second<<"\n";
            return memo->find(target)->second;
        }
        if (target == "") return 1;

        int totalCount = 0;
        
    for(auto i = 0;i < 10; i++)
    {   
        if (target.find(m[i]) == 0){
            
            int numOfWays = SumNum(target.substr(m[i].length()),m,memo);
            totalCount += numOfWays;
            
        }
    }
    memo->emplace(target, totalCount);
    return totalCount;
}
int main(){
    string ar[10] = {"e","ee","eee","eeee","eeeee","eeeeee","f","fdc","d","c"};
    string target = "eeeeeeeeeeeeefdc";
    auto answer = SumNum(target,ar);
    cout << answer;
}