factorial Factorial (階乗)

階乗 （nの階乗＝n!）

factorial Factorial について

recur_factorial.py

def recur_factorial(n):
  if n == 1:
      return n
  else:
      return n*recur_factorial(n-1)

num = 7

# check if the number is negative
if num < 0:
  print("Sorry, factorial does not exist for negative numbers")
elif num == 0:
  print("The factorial of 0 is 1")
else:
  print("The factorial of", num, "is", recur_factorial(num))

Memo

time complexities O(n!)
Use stack Data Stracture
recursive funnction

n Factorial	n (n - 1) (n - 2) ....1	n! = n × (n - 1)!	Result
1 Factorial	1	1	1
2 Factorial	2 × 1	= 2 × 1!	= 2
3 Factorial	3 × 2 × 1	= 3 × 2!	= 6
4 Factorial	4 × 3 × 2 × 1	= 4 × 3!	= 24
5 Factorial	5 × 4 × 3 × 2 × 1	= 5 × 4!	= 120

Fomula

n! = n \times (n - 1) \times (n - 2) \times \ldots \times 2 \times 1