int curIdx = 0;

private Node Tree31(int curDeep, int n, int l, int[] arr)
{
    if (curDeep >= l || curIdx>=n) return null;
    Node thisNode = new Node();
    thisNode.Value = arr[curIdx++];

    if (curDeep >= l || curIdx>=n) return thisNode;
    Node leftNode = Tree31(curDeep+1, n, l, arr);

    thisNode.Left = leftNode;

    if (curDeep >= l || curIdx>=n) return thisNode;
    Node rightNode = Tree31(curDeep+1, n, l, arr);

    thisNode.Right = rightNode;
    return thisNode;
}