题解 | #24点运算#

#include<bits/stdc++.h>
using namespace std;
map<string, int> c2n = {{"A", 1}, {"2", 2}, {"3", 3}, {"4", 4}, {"5", 5}, {"6", 6},{"7", 7}, {"8", 8}, {"9", 9}, {"10", 10}, {"J", 11}, {"Q", 12}, {"K", 13}};
map<int, string> n2c = {{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"}};
vector<char> Op = {'+', '-', '*', '/'};
vector<int> n(4);
vector<int>ops;
int cal(int a, int b, int op)
{
    if(op == 0) return a + b;
    else if (op == 1) return a - b;
    else if (op == 2) return a * b;
    else return a / b;
}
bool dfs(int start, int sum,vector<int>& ops)
{
    sum = cal(sum, n[start], *(ops.end()-1));
    if(start == 3)
        if(sum == 24) return true;
        else  return false;
    for(int i = 0; i < 4; i++)
    {
        ops.push_back(i);
        if (dfs(start + 1, sum,ops))
            return true;
        ops.pop_back();
    }
    return false;
}
int main()
{
    vector<string> s(4);
    cin >> s[0] >> s[1] >> s[2] >> s[3];
    for(int i = 0; i < 4; i++)
    {
        if(s[i] == "joker" || s[i] == "JOKER")
        {
            cout << "ERROR" << endl;
            return 0;
        }
        n[i] = c2n[s[i]];
    }
    sort(n.begin(), n.end());
    do
    {
        vector<int>ops;
        for (int i = 0; i < 4; i++)
        {
            ops.push_back(i);
            if (dfs(1, n[0], ops))
            {
                cout<<n2c[n[0]]<<Op[ops[0]]<<n2c[n[1]]<<Op[ops[1]]<<n2c[n[2]]<<Op[ops[2]]<<n2c[n[3]]<<endl;
                return 0;
            }
            ops.pop_back();
        }
    } while(next_permutation(n.begin(), n.end()));
    cout << "NONE" << endl;
}