测试数据结果一样，为啥WA

#include <iostream>
using namespace std;
int f[25][25][25];

int fun(int a, int b, int c) {
    if (a <= 0 || b <= 0 || c <= 0)
        return 1;
    else if (a > 20 || b > 20 || c > 20)
        return fun(20, 20, 20);
    else if (f[a][b][c] != 0)
        return f[a][b][c];
    else if (a < b && b < c)
        f[a][b][c] = fun(a, b, c - 1) + fun(a, b - 1, c - 1) - fun(a, b - 1, c);
    else
        f[a][b][c] = fun(a - 1, b, c) + fun(a - 1, b - 1, c) + fun(a - 1, b, c - 1) - fun(a - 1, b - 1, c - 1);
    return f[a][b][c];
}

int main() {
    while (1) {
        int a, b, c;
        cin >> a >> b >> c;
        if (a == -1 && b == -1 && c == -1)
            break;
        cout << "fun(" << a << ", " << b << ", " << c << ") = " << fun(a, b, c) << endl;
    }
}