Originally posted at 11/22/2010

This method has a bug, a very subtle one. Can you figure it out?

public IBinarySearchTree<TKey, TValue> TryRemove(TKey key, out bool removed, out TValue value)
{
    IBinarySearchTree<TKey, TValue> result;
    int compare = comparer.Compare(key, theKey);
    if (compare == 0)
    {
        removed = true;
        value = theValue;
        // We have a match. If this is a leaf, just remove it 
        // by returning Empty.  If we have only one child,
        // replace the node with the child.
        if (Right.IsEmpty && Left.IsEmpty)
            result = new EmptyAVLTree<TKey, TValue>(comparer, deepCopyKey, deepCopyValue);
        else if (Right.IsEmpty && !Left.IsEmpty)
            result = Left;
        else if (!Right.IsEmpty && Left.IsEmpty)
            result = Right;
        else
        {
            // We have two children. Remove the next-highest node and replace
            // this node with it.
            IBinarySearchTree<TKey, TValue> successor = Right;
            while (!successor.Left.IsEmpty)
                successor = successor.Left;
            result = new AVLTree<TKey, TValue>(comparer, deepCopyKey, deepCopyValue, successor.Key, 
                successor.Value, Left, Right.TryRemove(successor.Key, out removed, out value));
        }
    }
    else if (compare < 0)
        result = new AVLTree<TKey, TValue>(comparer, deepCopyKey, deepCopyValue, 
            theKey, theValue, Left.TryRemove(key, out removed, out value), Right);
    else
        result = new AVLTree<TKey, TValue>(comparer, deepCopyKey, deepCopyValue,
            theKey, theValue, Left, Right.TryRemove(key, out removed, out value));
    return MakeBalanced(result);
}