import sys
from collections import deque


def flatten(nested_list):
    """Flatten a list of nested lists of integers into a flat list of integers."""
    result = []
    nested_deque = deque(nested_list)

    while nested_deque:
        sublist = nested_deque.popleft()

        if isinstance(sublist, list):
            nested_deque.extendleft(reversed(sublist))
        else:
            result.append(sublist)
    return result


if __name__ == "__main__":
    assert flatten([]) == []
    assert flatten([1]) == [1]
    assert flatten([1, 2]) == [1, 2]
    assert flatten([1, [2]]) == [1, 2]
    assert flatten([1, [2, [3]]]) == [1, 2, 3]
    assert flatten([[[[1]]], [[[2]]], [[[3]]]]) == [1, 2, 3]

    # verify a list with depth greater than python's recursion limit
    # recursive flattening wouldn't pass this test:
    deeply_nested_list = [1, 2, 3]
    for i in range(sys.getrecursionlimit() + 1):
        deeply_nested_list[0] = [deeply_nested_list[0]]
    assert flatten(deeply_nested_list) == [1, 2, 3]

    # long nested list:
    assert flatten([[[[[i]]]] for i in range(100000)]) == list(range(100000))