class Solution:
    def levelOrder(self, root: TreeNode) -> List[List[int]]:
        if root == None:
            return []
        result, current_level = [], [root]

        # while there are nodes in the current_level, meaning while we haven't reached the end
        while current_level:
            next_level = []
            current_vals = []

            for node in current_level:
                # build the array of current level values to be appended to result
                if node != None:
                    current_vals.append(node.val)
                    # we get the next_level from each current_level
                    if node.left:
                        next_level.append(node.left)
                    if node.right:
                        next_level.append(node.right)
            result.append(current_vals)
            # and then set the current_level to next_level to move on
            current_level = next_level

        return result