2_1

def f21(arr):
    tree = {
        '_iter': 1,
        2012: {
            '_iter': 0,
            'rouge': {
                '_iter': 2,
                1971: 0,
                1965: 1
            },
            'fish': 2,
            'yaml': {
                '_iter': 2,
                1971: 3,
                1965: 4
            }
        },
        1970: {
            '_iter': 3,
            'scala': 5,
            'http': {
                '_iter': 0,
                'rouge': 6,
                'fish': 7,
                'yaml': 8
            }
        },
        1957: {
            '_iter': 3,
            'scala': 9,
            'http': 10
        }
    }

    while len(arr) > 0:
        _iter = tree['_iter']
        key = arr[_iter]
        if key not in tree:
            return None
        if isinstance(tree[key], dict):
            tree = tree[key]
        else:
            return tree[key]
    return None


print(f21(['rouge', 2012, 1965, 'http']))