public boolean isSameTree(TreeNode p, TreeNode q) {
        if (p ==null&&q ==null){
            return true;
        }
        if (p ==null||q == null){
            return false;
        }
        if (isSameTree(p.left,q.left)&&isSameTree(p.right,q.right)&&p.val == q.val){
            return true;
        }
        else {
            return false;
        }
    }