Binary search tree

Hi all,

Given a binary search tree and a number n, how can we find the
smallest node of the tree greater than n and the largest node smaller
than n?
For example for the following binary search tree where 5 is root and 4
is the left and 6 the right child, for an input of 5,
4 and 6 will be output:

5
4 6

Thanks,
Sara
.