Definition of mirror symmetry of a binary tree

It is necessary to check whether the given binary tree has mirror symmetry relative to its central axis.

class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;

    TreeNode() {}
    TreeNode(int val) { this.val = val; }
    TreeNode(int val, TreeNode left, TreeNode right) {
        this.val = val;
        this.left = left;
        this.right = right;
    }
}

public boolean isSymmetric(TreeNode root) {
    // TODO
    return false;
}