I got a list [1, [2, [3, [4, [5, [6, [7], 6], 5], 4], 3], 2], 1] and I wanna convert it to a single-dimensional list as the same order as 
it defined   i.e [1,2,3,4,5,6,7,6,5,4,3,2,1] 
here is my code, but it is actually not meet my request
list_1 = [1, [2, [3, [4, [5, [6, [7], 6], 5], 4], 3], 2], 1]
def my_solution(lst):
    for index, value in enumerate(lst):
        if isinstance(value, list):
            my_solution(value)
            lst.extend(value)
            del(lst[index])
    return lst
print(my_solution(list_1))
the output is [1, 1, 2, 2, 3, 3, 4, 4, 5, 5, 6, 6, 7]
