使用递归求解方程的建议

Question

所以！代表此等式中的阶乘（ps试图使用递归在java中求解）

等式为C（n，r）=n！ /（r！（n-r）！）This is the eqaution how you would write it normally (image is more clear)

注意：我尝试将for循环与递归组合在一起，但它始终会回答0，当我输入2和9时我希望它给出36的答案以下是我当前正在使用的代码

public long Recursion(long c){
    if (c <= 2) {
        return c;


    }
    return c*Recursion(c-1);

    //this assignment is my punishment for not using recursion
}
    private void CalMouseClicked(java.awt.event.MouseEvent evt) {                                 
        // TODO add your handling code here:
          int n = Integer.parseInt(Var_n.getText());
        int r = Integer.parseInt(var_r.getText());
        long Ans = 0;

        for(int i = 2; i <= n; i++){
            for(int y=2;y <=r;y++){
                long n2 = Recursion(n);
                 System.out.println(String.valueOf(n2));
                long r2 = Recursion(r);
                 System.out.println(String.valueOf(r2));
                 long g = (n-r);
                 System.out.println(String.valueOf(g));
                 long x = r2*g;
                Ans = n2/(Recursion(x));

 System.out.println(String.valueOf(Ans));
        Output.setText("There are " + String.valueOf(Ans) + " ways");
    }                                

Here's a screenshot of the output along with my code(which I've since edited)

编辑：我固定了将r代替r2的错字，但没有改变ans

Answer 1

             long g = (n-r);
             System.out.println(String.valueOf(g));
             long x = r2*g;
             Ans = n2/(Recursion(x));

这部分不正确。请记住，公式是n! / r!(n-r)!

您已经计算出n-r，因此需要计算其阶乘，然后乘以r2。您要乘以r2 *（n-r），然后乘以全部的阶乘。

代码应显示为

Ans = n2/r2*(Recursion(g));