题解 | #24点运算#

//递归（深度优先搜索）+map

#include<vector>
#include<unordered_map>
using namespace std;
vector<int> num2;
string temp;
vector<bool> signal(4);
bool begin1 = false;
void dfs(vector<int> tp,int n, int sum, string ans, vector<int> num){
    if(n==4 && sum == 24){
        begin1 = true;
        temp = ans;
        num2 = num;
        return;
    }
    if(n == 4 || begin1) return;
    for(int i =0 ; i < 4 ; i++ ){
        if(!signal[i]){
            signal[i] = true;
            num.emplace_back(tp[i]);
            dfs(tp, n + 1, sum + tp[i], ans + "+",num );
            dfs(tp, n + 1, sum - tp[i], ans + "-",num );
            dfs(tp, n + 1, sum * tp[i], ans + "*",num );
            dfs(tp, n + 1, sum / tp[i], ans + "/",num );
            signal[i] = false;
            num.pop_back();
        }
    } 
}
int main(){
    vector<int> tp;
    unordered_map<int, char> mp = {{1,'A'},{2,'2'},{3,'3'},{4,'4'},{5,'5'},{6,'6'},
                                   {7,'7'},{8,'8'},{9,'9'},{10,'10'},{11,'J'},{12,'Q'},{13,'K'}};
    string str;
    while(cin >> str){
        if(str == "JOKER" || str == "joker"){
            cout << "ERROR" << endl;
            return 0;
        }else if(str == "10"){
            tp.emplace_back(10);
        }else if(str == "J"){
            tp.emplace_back(11);
        }else if(str == "Q"){
            tp.emplace_back(12);
        }else if(str == "K"){
            tp.emplace_back(13);
        }else if(str == "A"){
            tp.emplace_back(1);
        }else{
            tp.emplace_back(str[0] - '0');
        }
    }
    vector<int> num;
    string ans;
    for(int i =0 ; i < 4 ; i++ ){
        signal[i] = true;
        num.emplace_back(tp[i]);
        dfs(tp, 1, tp[i], ans ,num);
        signal[i] = false;
        num.pop_back();
    }
    if(begin1) cout<<mp[num2[0]]<<temp[0]<<mp[num2[1]]<<temp[1]<<mp[num2[2]]<<temp[2]<<mp[num2[3]]<<endl;
    else cout<<"NONE"<<endl;
    return 0;
    
}