Python: Get all strobogrammatic numbers that are of length n

Python Basic - 1: Exercise-17 with Solution

Write a Python program to get all strobogrammatic numbers that are of length n.

A strobogrammatic number is a number whose numeral is rotationally symmetric, so that it appears the same when rotated 180 degrees. In other words, the numeral looks the same right-side up and upside down (e.g., 69, 96, 1001).

For example,
Given n = 2, return ["11", "69", "88", "96"].
Given n = 3, return ['818', '111', '916', '619', '808', '101', '906', '609', '888', '181', '986', '689']

Pictorial Presentation:

Sample Solution:

Python Code:

#https://github.com/keon/algorithms/blob/master/math/generate_strobogrammtic.py
def gen_strobogrammatic(n):
    """
    :type n: int
    :rtype: List[str]
    """
    result = helper(n, n)
    return result


def helper(n, length):
    if n == 0:
        return [""]
    if n == 1:
        return ["1", "0", "8"]
    middles = helper(n-2, length)
    result = []
    for middle in middles:
        if n != length:
            result.append("0" + middle + "0")
        result.append("8" + middle + "8")
        result.append("1" + middle + "1")
        result.append("9" + middle + "6")
        result.append("6" + middle + "9")
    return result

print("n = 2: \n",gen_strobogrammatic(2))
print("n = 3: \n",gen_strobogrammatic(3))
print("n = 4: \n",gen_strobogrammatic(4))

Sample Output:

n = 2: 
 ['88', '11', '96', '69']
n = 3: 
 ['818', '111', '916', '619', '808', '101', '906', '609', '888', '181', '986', '689']
n = 4: 
 ['8008', '1001', '9006', '6009', '8888', '1881', '9886', '6889', '8118', '1111', '9116', '6119', '8968', '1961', '9966', '6969', '8698', '1691', '9696', '6699']

Flowchart:

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