16. BFS Level Order

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Input: root = [3,9,20,null,null,15,7]

Output: [[3],[9,20],[15,7]]

Explanation: The tree has 3 levels: the root (3), the children of the root (9, 20), and the children of 9 and 20 (15, 7).

Input: root = [1]

Output: [[1]]

Explanation: The tree has only one node, the root (1).

Input: root = []

Output: []

Explanation: The tree is empty, so the result is an empty list.

Time Complexity

O(n)

Space Complexity

O(w)