freeing memory of a binary tree C

Since it's a tree, you should go with a recursive approach.

deallocate (node):
    //do nothing if passed a non-existent node
    if node is null
        return

    //now onto the recursion
    deallocate(left node)
    deallocate(right node)

    free node

Think about what the different traversal types do, and keep in mind, after you free memory you're not allowed to access it anymore:

• Preorder: operation performed before visiting any children
• In-order: operation performed after visiting left subtree, before right subtree
• Postorder: operation performed after visiting all subtrees

Given the above statements, the answer should be clear.