使用递归，完成传入n，返回1! + 2! + 3! + ... + n!

# n => 1! + 2! + ... (n-1)! + n!
# f(n) => f(n-1) + n!
# f(n) => f(n-1) + n * (n-1)!
# f(n) => f(n-1) + n * (f(n-1)-f(n-2))
def jiechenghe(n):
    if n == 1:
        return 1
    elif n == 2:
        return 3
    return jiechenghe(n-1) + n * (jiechenghe(n-1)-jiechenghe(n-2))
print(jiechenghe(1))
print(jiechenghe(2))
print(jiechenghe(3))