def odd(n):
    #return n%2 == 1
    if n == 0:
        return False
    return even(n-1)

def even(n):
    #return n%2 == 0
    if n == 0:
        return True
    return odd(n-1)

odd(4)

False

odd(3) = even(2) = odd(1) = even(0) = True

def insert(x,nums):
    #i = 0
    #while i < len(nums) and x >= nums[i]:
    #    i = i + 1
    #return nums[0:i] + [x] + nums[i:]
    #BASE CASE - do not forget
    if nums == []:
        return [x]
    if x <= nums[0]:
        return [x] + nums
    return [nums[0]] + insert(x,nums[1:])
    

def isort(nums):
    if nums == []:
        return []
    return insert(nums[0],isort(nums[1:]))

isort([20,10,30])

[10, 20, 30]