Here is source code of the C Program to Find the Smallest and Largest Elements in the Binary Search Tree. The C program is successfully compiled and run on a Linux system. The program output is also shown below.

/*
* C Program To Find the Smallest and Largest Elements
* in the Binary Search Tree
* 40
* / \
* 20 60
* / \ \
* 10 30 80
* \
* 90
* (Given Binary Search Tree)
*
*/
#include <stdio.h>
#include <stdlib.h>

struct btnode
{
int value;
struct btnode *l;
struct btnode *r;
}*root = NULL;

typedef struct btnode N;
N* new(int);
void create();
void preorder(N *t);
void min_max(N *t);

void main()
{
int choice;

create();
while (1)
{
printf("Enter the choice\n");
printf("1-Display : 2-Min & Max element : 3-Exit\n");
scanf("%d", &choice);
switch (choice)
{

case 1:
printf("preorder preorder of tree elements\n");
preorder(root);
printf("\n");
break;
case 2:
min_max(root);
break;
case 3:
exit(0);
default:
printf("Enter the right choice\n");
}
}
}

/* creating temporary node */
N* new(int data)
{
N* temp = (N*)malloc(sizeof(N));
temp->value = data;
temp->l = NULL;
temp->r = NULL;

return(temp);
}

/* Creating the binary search tree */
void create()
{
root = new(40);
root->l = new(20);
root->r = new(60);
root->l->l = new(10);
root->l->r = new(30);
root->r->r = new(80);
root->r->r->r = new(90);
}

/* To display preorder traversal of the tree */
void preorder(N *temp)
{
printf("%d->", temp->value);
if (temp->l != NULL)
preorder(temp->l);
if (temp->r != NULL)
preorder(temp->r);
}

/* TO find the minimum and maximum values in the given tree */
void min_max(N *temp)
{
while (temp->l != NULL)
temp = temp->l;
printf("Minimum value = %d\n", temp->value);
temp = root;
while (temp->r != NULL)
temp = temp->r;
printf("Maximum value = %d\n", temp->value);
}


$ cc tree9.c
$ a.out
Enter the choice
1-Display : 2-Min & Max element : 3-Exit
1
preorder preorder of tree elements
40->20->10->30->60->80->90
Enter the choice
1-Display : 2-Min & Max element : 3-Exit
2
Minimum value = 10
Maximum value = 90
Enter the choice
1-Display : 2-Min & Max element : 3-Exit
3