private TreeNode convert(int[] num, int low, int high)
    {
        if(low > high)
            return null;
        int mid = low + (high - low)/2;
        int mid_val = num[mid];
        TreeNode node = new TreeNode(mid_val);
        node.left = convert(num, low, mid-1);
        node.right = convert(num, mid+1, high);
        
        return node;
    }