algospy

flatten

Implement Flatten Arrays. Given an array that may contain nested arrays, produce a single resultant array.

Knowledge of isinstance() is the key. Then a simple recursion logic works. Traverse over elements in a list, and if the elements are lists themselves, we need to get into nested loops. So this calls for a recursion stack. A recursive method requires an initialiser wrapper, but with Python’s capability of using optional arguments, we can finish this using a single function.

def flatten(l, answer=[]):
    for elem in l:
        if not isinstance(elem, list):
            answer.append(elem)
        else:
            flatten(elem, answer)
    return answer

But there is a problem here. The problem with using default arguments is that only one instance of them actually exists. So, we need answer to be initialised with an empty list every time this function is called.

def flatten(l, answer=None):
    if answer is None:
        answer = []
    for elem in l:
        if not isinstance(elem, list):
            answer.append(elem)
        else:
            flatten(elem, answer)
    return answer

I saw from this SO question that one can also do answer = None or answer. Cool.