递归

  boolean isSymmetrical(TreeNode pRoot) {
        if (pRoot==null)
            return true;
        return judge(pRoot.left,pRoot.right);
    }
    public boolean judge(TreeNode root1,TreeNode root2){

        if (root1==null && root2==null)
            return true;
        else if(root1==null||root2==null){
            return false;
        }else if (root1.val!=root2.val)
            return false;
        else
            return judge(root1.left,root2.right) && judge(root1.right,root2.left);
    }