Java 求组合数C(n,m)

//从n个球中取m个的组合种类
public static int C(int n,int m){
    if(n==m)
        return 1;
    else
        return  JC(n)/(JC(m)*JC(n-m));
    }
//求阶乘
    public static int JC(int num){
        if (num==1){
            return 1;
        }else {
            return JC(num-1)*num;
        }
    }

public static int sumOfC(int num){
        int sum = 0;
        if (num == 1){
            return 1;
        }else {
            for (int i = 1; i <= num; i++) {
                sum+=C(num,i);
            }
            return sum;
        }
    }