Problems/102. Binary Tree Level Order Traversal
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102. Binary Tree Level Order Traversal

MediumBinary TreeTreeBFSQueue

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]]
Level 0 has node 3. Level 1 has nodes 9 and 20. Level 2 has nodes 15 and 7.
Example 2
Input: root = [1, null, 2, null, 3]
Output: [[1], [2], [3]]
Since each node only has a right child, the tree is skewed. Each level has exactly one node.
Example 3
Input: root = []
Output: []
The tree is empty, so we return an empty list.
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