Description
Primary Learning Objectives
• Write a C program that works with binary search trees (BSTs).
Requirements
As starter code for this assignment, use the C code contained in as7_starter_code.zip file found
on Avenue to Learn accompanying this assignment.
This starter code contains the same binary search tree type used in lectures and labs, and the
same set of functions for working with binary search trees that we created together in lecture
and lab. The starter code also contains the following function declarations:
// HINT: This function was used as a helper function for array_of_sorted_keys
// in the instructor’s solution:
// void build_array(bstNode *node, int *array, int *current_index);
int *array_of_sorted_keys(bstNode *node);
// HINT: This function was used as a helper function for
// balanced_tree_from_sorted_array in the instructor’s solution:
// bstNode *construct_tree(int *array, int min, int max);
bstNode *balanced_tree_from_sorted_array(int *sorted_array, int length);
bstNode *balanced_tree_copy(bstNode *tree);
At a minimum you will need to implement the array_of_sorted_keys,
balanced_tree_from_sorted_array, and balanced_tree_copy functions. Two function
declarations have been commented out in the starter code, but they were used by the
instructor to create their solution to the problems, and so you can implement these as well if
you find them helpful (I would recommend implementing them).
The array_of_sorted_keys function must accept as an argument the head node of a BST. It
needs to return a pointer to a dynamically allocated array that contains the keys of the BST
sorted in ascending order. In order to do this, you’ll need to dynamically allocate an array large
enough to hold all of the keys. The num_nodes function can help you with this. In the week 9
lab you’ll learn that you can print out the keys of a BST in an ascending sorted order if you do an
in-order traversal of the tree. In this case, instead of printing the nodes, you’ll need to store
them in the dynamically allocated array that you’ve created.
In the instructor’s solution, they used a helper function void build_array(bstNode *node, int
*array, int *current_index) which performed an in-order traversal of the BST, storing the
results into array, and using current_index to keep track of the index to place the next element
visited into the array. You don’t have to solve the problem exactly this way, but this way is
recommended, and whatever you do, do not use a global variable to manage this.
The balanced_tree_from_sorted_array function needs to accept as arguments an array of ints
stored in ascending sorted order, and the length of that array. It needs to return a pointer to a
newly created balanced binary search tree that contains as keys all of the elements in the array
(i.e. you are converting the array contents to a balanced binary search tree). A balanced binary
search tree has the minimum height necessary to contains all of the keys. If you wanted to
create a balanced tree from a sorted array, then the best node for the root node would be the
middle element of the array, given that half the array elements will be less than this element
and half the array elements will be greater than this element. If the array contains an even
number of elements either of the two ‘middle’ elements will work. If you recursively applied
this logic to each remaining left and right subsection of the array to build the left and right
subtrees of this node, you could build a balanced binary search tree. So a function could work
like this:
1. Is there still a middle to be found? If not, return NULL.
2. Get the middle of the array, create a new node with this value as the key.
3. Take the left half of the array, call the function recursively and set the left child of the
node to the result.
4. Take the right half of the array, call the function recursively and set the right child of the
node to the result.
5. Return the node.
In order to do this, the instructor’s solution used a helper function bstNode *construct_tree(int
*array, int min, int max) where min is the beginning index of the segment of the array being
converted, and the max is the ending index of the segment of the array being converted. By
recursively calling construct_tree with min and max values that reflect the portion of the array
being converted for a given subtree, we can follow the above algorithm. We detect that there
is no longer a “middle” that can be found when the function is called with a min > max. You do
not need to solve the problem exactly this way, but this is the recommended approach.
The balanced_tree_copy function should accept as input a binary search tree that is potentially
unbalanced. It should return a copy to a new balanced binary search tree on the heap which
contains the same keys. In order to do this, you can call the array_of_sorted_keys function to
obtain an array containing the keys of the binary search tree in ascending sorted order. Then
you can call balanced_tree_from_sorted_array to obtain a balanced binary search tree from
these results. Don’t forget to free the array you created as there is no need for it after the
function is done!
Test Code
The main function contains test code to test the 3 functions. You may want to “comment out”
this code by surrounding it with /* … */ multiline comments so that you can work on your
functions, as the code assumes that all of the functions have been implemented. You can use
this code to help verify that your functions are working correctly.
The expected test code results are provided after the mark breakdown, and as a plaintext file
on Avenue to Learn accompanying the assignment.
Submission
Save your solution in a file named bst.c and put it in a zip file named a7.zip and submit it to the
Assignment #7 dropbox on Avenue to Learn.
Marking scheme
Component Description Marks
array_of_sorted_keys Are the keys of the BST stored into an array
that is returned? Are they sorted in
ascending order? Is it don’t without a
40
global variable? Is it done with an in-order
traversal of the list? Is it done reasonably
efficiently and in a logical way?
balanced_tree_from_sorted_array Is a balanced tree created from the array?
Does it conform to a BST? Is it done
reasonably efficiently and in a logical way?
45
balanced_tree_copy Is a balanced tree returned that contains all
the same keys as the unbalanced tree? Are
the two functions above used to do so?
15
Total: 100
Test Code Results
Tree before converted into a sorted array:
8
3
1
6
4
7
10
14
13
Array: 1 3 4 6 7 8 10 13 14
Tree constructed from the array:
7
3
1
4
6
10
8
13
14
Print out of an unbalanced tree:
0
1
2
3
4
5
6
7
8
9
Print out of the now balanced tree:
4
1
0
2
3
7
5
6
8
9
