Binary Tree Zigzag Level Order Traversal
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).
Input: root = [3,9,20,null,null,15,7]
- Time Complexity: O(N), where N is the number of nodes in the tree.
- Space Complexity: O(H), where H is the height of the tree, i.e. the number of levels in the tree, which would be roughly log2N.