Binary Tree Level Order Traversal

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).

Examples

Example 1:

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

      3
     / \
    9  20
       / \
      15   7

Example 2:

Input: root = [1]
Output: [[1]]

Example 3:

Input: root = []
Output: []

Constraints

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

Expected Complexity

• Time: O(n)
• Space: O(n)