176. Check if Binary Tree is Symmetric

Write a function solve that checks if a binary tree is symmetric around its center. The tree is represented as a list where None represents a missing node.

Example:
Input: [1, 2, 2, 3, 4, 4, 3]
Output: true

Example 2:
Input: [1, 2, 2, null, 3, null, 3]
Output: false

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