问题描述:
Write a function to find the demlo number for the given number.

测试用例:
['assert find_demlo("111111") == \'12345654321\'', 'assert find_demlo("1111") == \'1234321\'', 'assert find_demlo("13333122222") == \'123456789101110987654321\'']

生成的代码:
def find_demlo(num_str):
    n = len(num_str)
    max_digit = n // 2 + 1
    ascending = ''.join(str(i) for i in range(1, max_digit + 1))
    descending = ''.join(str(i) for i in range(max_digit - 1, 0, -1))
    return ascending + descending

# Test cases
assert find_demlo("111111") == '12345654321'
assert find_demlo("1111") == '1234321'
assert find_demlo("13333122222") == '123456789101110987654321'