null也是镜像对称的树？我佛了

牛客的IDE有些地方真实睿智，凭什么一个null也是镜像对称的。。

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