Python 알고리즘_1부터 n까지의 합 구하기

# algorithm_1, 계산복잡도 : O(n)
def sum_num(n):
    result = 0
    for i in range(n + 1):
        result += i
    return result

# sum_num_recursion_algorithm, 계산복잡도 : O(n)
def sum_num_recursion(n):
    if n < 0:
        return print('Please enter only positive numbers')
    elif n == 0:
        return 0
    return n + sum_num_recursion(n - 1)

# algorithm_2, 계산복잡도 : O(1)
def sum_num2(n):
    return (n + 1) * (n / 2)