Binary Tree Level Order Traversal

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).

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Example 1

    3
   / \
  9  20
    /  \
   15   7

Input: root = [3, 9, 20, null, null, 15, 7]
Output: [[3], [9, 20], [15, 7]]
Explanation: Three levels - root alone, then its two children, then the two grandchildren.

Example 2

Input: root = [1]
Output: [[1]]
Explanation: Single node, one level.

Example 3

Input: root = []
Output: []
Explanation: Empty tree has no levels.

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Constraints

• The number of nodes in the tree is in the range [0, 2000].
• -1000 <= Node.val <= 1000