Problem - Symmetric Tree

Given the root of a binary tree, check whether it is a mirror of itself (symmetric around its center). Example: $[1,2,2,3,4,4,3]$ is symmetric. $[1,2,2,null,3,null,3]$ is not.

This problem combines ideas from "same tree" and "invert tree." A tree is symmetric if its left subtree is a mirror of its right subtree. Constraints: up to $1000$ nodes, values from $-100$ to $100$.