RECURSION

for i in range(5,10):
    print(i)

5
6
7
8
9

factorial(n) = 123...(n-1)*n

def factorial1(n):
    if n < 0:
        return None
    result = 1
    for i in range(2,n+1):
        result = result * i
    return result

factorial1(10)

3628800

def factorial2(n):
    if n < 0:      # BASE CASE
        return None
    elif n <= 1:   # BASE CASE
        return 1
    else:
        return n*factorial2(n-1)   # RECURSIVE CASE

factorial2(10)

3628800

def sum_list1(xs):
    result = 0
    for i in xs:
        result = result + i
    return result 

sum_list1([20,10,30])

60

def sum_list2(xs):
    if xs == []:
        return 0
    else:
        return xs[0] + sum_list2(xs[1:])

sum_list2([20,10,30])

60