public class RecursionTest {
  public static void main(String[] args) {
    System.out.println(diGui(100));// 5050
  }
  public static int diGui(int n) {
    int sum;
    if (n == 1)
      return 1;
    else {
      sum = n + diGui(n - 1);
      return sum;
    }
  }
}

public class RecursionTest {
  public static void main(String[] args) {
    System.out.println(diGui(5));// 120 注意如果求100的阶乘那么就不能用int或者long,计算后的结果数值过大程序无法返回，一般情况会返回0要用BigInteger
  }
  public static int diGui(int n) {
    int sum;
    if (n == 1)
      return 1;
    else {
      sum = n * diGui(n - 1);
      return sum;
    }
  }
}