**Problem Statement:**
Given the required input arguments, write an efficient algorithm to solve the **Binary Tree Zigzag Level Order** problem. Implement the required logic as specified by standard definitions for this classic algorithmic challenge.
**Hint / Expected Approach:**
BFS + alternating deque direction per level
**Edge Cases to Consider:**
- (1) Single node
- (2) All left children
- (3) Perfectly symmetric tree
Examples
Example 1
Input:[3,9,20,null,null,15,7]
Output:[[3],[20,9],[15,7]]
Explanation: Zigzag classic
Example 2
Input:[1]
Output:[[1]]
Explanation: Single node
Example 3
Input:[]
Output:[]
Explanation: Empty
Constraints
▪The number of nodes in the tree is in the range [0, 2000]
▪-1000 <= Node.val <= 1000
▪Ensure depth-first or breadth-first traversals are memory-efficient