recursion.py

# in: [1, 2, 3]
# out: [1, 2, 3]

# in: [1, [1, 2], 2, [3]]
# out: [1, 1, 2, 2, 3]

# Hint: You’ll need to use a for-loop somewhere.

def flatten(l, new_list = None):
	# Base Case
	new_list = new_list or []
	for item in l:
		if type(item) == list:
			flatten(item, new_list)
		else:
			new_list.append(item)
	return new_list
	# Return and calling flatten again

print(flatten([1, [1, 2], 2, [3]]))


# Call 1 ([1, [1, 2], 2, [3]], none)
# For loop: [1, [1, 2], 2, [3]]
# index 1, int, append
# index 2, list, flatten

# Call 2: ([1, 2], [1])
# For loop: [1, 2]
# index 1, int, append
# index 2, int, append

# Call 1:
# index 3, int, append
# index 4, list, flatten

# Call 2: ([3], [1, 1, 2, 2])
# index 1, int, append

# Call 1:
# return [1, 1, 2, 2, 3]