Binary Tree Symmetry

Given the root of a binary tree, determine whether it is a mirror of itself — that is, the left subtree is a mirror reflection of the right subtree.

Example 1

Input: [1, 2, 2, 3, 4, 4, 3]

         1
       /   \
      2     2
     / \   / \
    3   4 4   3

Output: true.

Example 2

Input: [1, 2, 2, null, 3, null, 3]

        1
      /   \
     2     2
      \     \
       3     3

Output: false — the two 3 nodes are both on the "inner" side relative to the mirror axis, so this is not symmetric even though both subtrees look superficially similar.

Example 3

Input: [] → Output: true (an empty tree is trivially symmetric).

Constraints

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