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Explanation: the shortcuts based on + (including the implied use in sum) are, of necessity, O(T**2) when there are T sublists -- as the intermediate result list keeps getting longer, at each step a new intermediate result list object gets allocated, and all the items in the previous intermediate result must be copied over (as well as a few new ones added at the end). So, for simplicity and without actual loss of generality, say you have T sublists of k items each: the first k items are copied back and forth T-1 times, the second k items T-2 times, and so on; total number of copies is k times the sum of x for x from 1 to T excluded, i.e., k * (T**2)/2.

The list comprehension just generates one list, once, and copies each item over (from its original place of residence to the result list) also exactly once.

Make a function which append all the elements of list of list into the flatten list.

def convert_flatten_list(lst_of_lst): 
	for i in lst_of_lst: 
		if(type(i)==list): 
			convert_flatten_list(i) 
		else: 
			flatten_lst.append(i) 
             
             
lst_of_lst = [1,2,[3,4,5,[6]],[7,[8,[9]]]] 
flatten_lst = []   
 
print("Original list:",lst_of_lst) 
convert_flatten_list(lst_of_lst) 
print("New list:",flatten_lst) 

Output:

Original list: [1, 2, [3, 4, 5, [6]], [7, [8, [9]]]]

New list: [1, 2, 3, 4, 5, 6, 7, 8, 9]

 

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