Given the root of a binary tree, check whether it is a mirror of itself (i.e., symmetric around its center).

Example 1:

Input:  1 2 2 3 4 4 3
Output: true

Example 2:

Input:  1 2 2 null 3 null 3
Output: false

Constraints:

• The number of nodes in the tree is in the range [1, 1000]
• -100 ≤ Node.val ≤ 100

Input format: BFS level-order, space-separated, null for missing nodes.

Output format: true or false.