Symmetric Tree

A tree is symmetric if it is a mirror image of itself around its centre (the left subtree mirrors the right subtree).

Tree format. A binary tree is given on one line in level order (top to bottom, left to right), with null for a missing child — the same way most interview platforms show trees. Trailing nulls are left out, and an empty tree is the single word null. Example: 3 9 20 null null 15 7 is

    3
   / \
  9   20
     /  \
    15   7

Input: one tree (at least one node).

Output: true or false.

Constraints: 1 ≤ nodes ≤ 10⁴. The tree has at most 10⁴ nodes and its height is at most 500, so recursion is safe in every language.

Think about it: two subtrees mirror each other when their roots are equal, left's left mirrors right's right, and left's right mirrors right's left.