对称二叉树

   public boolean isSymmetric(TreeNode root) {
        if(root == null) return true;
    //把一颗树的左右子树 看成独立的两棵树 去比较他俩是否对称
        return Symmetric(root.left,root.right);

    }
//判断是否对称
    public boolean Symmetric(TreeNode left, TreeNode right){
        if(left == null && right == null) return true;
        if(left != null && right != null && left.val == right.val){
            return Symmetric(left.left,right.right) && Symmetric(left.right , right.left);
        }
        return false;
    }