class Solution {
    public boolean isSymmetric(TreeNode root) {
        if (root == null) return true;
        return isSymmetric(root.left, root.right);
    }
    public boolean isSymmetric(TreeNode p, TreeNode q) {
        if (p == null && q == null) return true;
        if (p == null || q == null || p.val != q.val) return false;
        return isSymmetric(p.left, q.right) && isSymmetric(p.right, q.left);
    }
}