flatten list

def flatten(xs) {
    xs = xs?:[]
    def orig_list = new LinkedList(xs)
    def flat_list = []

    while (orig_list) {
        def elem = orig_list.pollFirst()
        switch (elem) {
            case List:
                elem.reverseEach { orig_list.offerFirst(it) }
                break
            default:
                flat_list << elem
        }
    }
    flat_list
}

def original = [[1,2,[3]],4]

assert flatten(original) == [1, 2, 3, 4]
assert flatten([]) == []
assert flatten([[[]],[[[]]],[]]) == []
assert flatten([0]) == [0]
assert flatten([0, [null]]) == [0, null]
assert flatten(null) == []