Level Order Traversal

Return the values level by level, from the top, and left to right inside each level. The result is a list of lists, one per level.

The tests write a tree as a level-order list (row by row, left to right) with None for a missing child: [3, 9, 20, None, None, 15, 7] is a root 3 with children 9 and 20, where 20 has children 15 and 7.

For [3, 9, 20, None, None, 15, 7] the answer is [[3], [9, 20], [15, 7]].

Examples

Example 1

Input:

level_order(root)  # root = [3, 9, 20, None, None, 15, 7]

Output:

[[3], [9, 20], [15, 7]]

Example 2

Input:

level_order(root)  # root = [1]

Output:

[[1]]

Constraints

• 0 <= number of nodes <= 100000
• -10**9 <= val <= 10**9