Floor by Floor

medium

A building inspector works a tree-shaped complex one floor at a time: everything on the current level gets logged before anyone climbs to the next. Her report groups rooms by floor, left to right.

Given the root of a binary tree, return its values grouped by depth: a list of levels, each level a left-to-right list of values.

Tests pass a level-order array; implement the core in _levels on real nodes.

Example 1

in values = [3, 9, 20, null, null, 15, 7]

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

Example 2

in values = [1]

out [[1]]

Example 3

in values = []

out []

Constraints

· 0 <= number of nodes <= 2000
· -1000 <= node value <= 1000