Write a Python program to flatten a given nested list structure

def flatten_list(n_list):
    result_list = []
    if not n_list: return result_list
    stack = [list(n_list)]
    while stack:
        c_num = stack.pop()
        next = c_num.pop()
        if c_num: stack.append(c_num)
        if isinstance(next, list):
            if next: stack.append(list(next))
        else: result_list.append(next)
    result_list.reverse()
    return result_list 
n_list = [0, 10, [20, 30], 40, 50, [60, 70, 80], [90, 100, 110, 120]]
print("Original list:") 
print(n_list)
print("\nFlatten list:")
print(flatten_list(n_list))