问二叉树是BST -为什么一个可以工作而另一个不能

EN

// This works - Implementation 1
--------------------------------
class PrevWrapper
{
    int data = Integer.MIN_VALUE;
}

public boolean isBST()
{
    return isBST(root, new PrevWrapper());
}

private boolean isBST(Node node, PrevWrapper previousElement)
{
    if ( node == null )
    {
        return true;
    }

    if ( !isBST(node.left, previousElement) )
    {
        return false;
    }

    if ( previousElement.data > node.item )
    {
        return false;
    }

    previousElement.data = node.item;
    return isBST(node.right, previousElement);
}

// This works - Implementation 2
--------------------------------
static int lastValue = Integer.MIN_VALUE;

static boolean isBST(Node root)
{
    if ( root == null )
    {
        return true;
    }
    if ( ! isBST(root.left) )
    {
        return false;
    }
    if ( root.item < lastValue )
    {
        return false;
    }
    lastValue = root.item;
    return isBST(root.right);
}

// This does not work - Implementation 3
--------------------------------

private boolean isBST(Node node, Integer previousElement)
{
    if ( node == null )
    {
        return true;
    }

    if ( !isBST(node.left, previousElement) )
    {
        return false;
    }

    if ( previousElement > node.item )
    {
        return false;
    }

    previousElement = node.item;
    return isBST(node.right, previousElement);
}