马老师的烦恼

// 0-9的阶乘
const int FAC[] = {1, 1, 2, 6, 24, 120, 720, 5040, 40320, 362880};

int cantor(int a[], int n) {
    int ans = 0;
    for (int i = 0; i < n; ++i) {
        int lessThan = 0;
        for (int j = i + 1; j < n; ++j) {
            if (a[j] < a[i]) lessThan++;
        }
        ans += lessThan * FAC[n - i - 1];
    }
    return ans;
}

void decode(int ans[], int x, int n) {
    bool visit[9] = {false};
    for (int i = 0; i < n; ++i) {
        ans[i] = x / FAC[n - i - 1];
        int j, order = 0;
        for (j = 0; j < n; ++j) {
            if (!visit[j]) {
                if (ans[i] == order) break;
                order++;
            }
        }
        ans[i] = j + 1;
        visit[j] = true;
        x %= FAC[n - i - 1];
    }
}