The problem. Given the root of a binary tree, return its values level by level, from top to bottom, and left to right within each level — as a list of lists, one list per level.
![The tree 3, 9, 20, null, null, 15, 7 with its levels shaded. Level 0 is [3], level 1 is [9, 20], level 2 is [15, 7]. The output is [[3], [9, 20], [15, 7]].](/drawings/097643b8-20d3-4c0e-9fa6-14edf97c65d9.svg)
Input: root = [3, 9, 20, null, null, 15, 7]
Output: [[3], [9, 20], [15, 7]]
Input: root = [1] Output: [[1]]
Input: root = [] Output: []Preorder, inorder and postorder all go deep first, so they jump between levels. Here we must finish a level before starting the next — this is breadth-first order.
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