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