Get biggest level

func levelOrder(_ root: TreeNode?) -> Int {
    guard let root = root else { return []}
    var queue = [TreeNode]()
    queue.append(root)
    var ans = 0
    while !queue.isEmpty{
        ans = max(ans, queue.count)
        for _ in 0 ..< queue.count {
            let cur = queue.removeFirst()
            if let l = cur.left { queue.append( l )}
            if let r = cur.right { queue.append( r )}
        }
    }
    return ans
}