Given the root of a binary tree, return the zigzag level order traversal of its nodes' values. (i.e., from left to right, then right to left for the next level and alternate between).
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Example 1:
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++Input: root = [3,9,20,null,null,15,7] +Output: [[3],[20,9],[15,7]] ++ +
Example 2:
+ ++Input: root = [1] +Output: [[1]] ++ +
Example 3:
+ ++Input: root = [] +Output: [] ++ +
+
Constraints:
+ +[0, 2000].-100 <= Node.val <= 100