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Given the root of a binary tree, return the level order traversal of its nodes' values. (i.e., from left to right, level by level).
Example 1
Input: root = [3,9,20,null,null,15,7]
Output: [[3],[9,20],[15,7]]
Explanation: Reading the tree one level at a time from left to right: level 0 is [3], level 1 is [9,20] and level 2 is [15,7].
Example 2
Input: root = [1]
Output: [[1]]
Explanation: The tree has a single node, so there is one level: [[1]].
Example 3
Input: root = []
Output: []
Explanation: The tree is empty, so there are no levels to return.
BFS naturally visits nodes one level at a time. The trick to grouping the output by level is to snapshot the *queue's size* at the start of each iteration — at that moment the queue contains exactly the nodes of one level. Drain that many, enqueue their children, and you've completed one level cleanly.
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