劉桂秀Java遞回函數recursive function, 遞回Recursion

public class Main {//*劉桂秀w3schools遞回函數Recursion//
  public static void main(String[] args) {
    int x =10;
    for (int i=0; i < x ; i++){  //*使用for迴圈//
    System.out.print( i + "累加等於 " +sum(i) );//*sum函數,參數x//
    System.out.println( "累乘等於 "+ f(i) ) ; }
  }
  public static int sum(int k) {//*定義sum函數,參數k//
    if (k > 0) {
      return k + sum(k - 1);   //*sum函數再呼叫sum,f(f(k))//
    } else {
      return 0;}
  }
    public static int f(int k) {//*定義sum函數,參數k//
    if (k > 0) {
      return k * f(k - 1);   //*sum函數再呼叫sum,f(f(k))//
    } else{
    return 1;
    } //*0!=1, 5!=5x4x3x2x1//
  }
}