A binary search tree is a tree where each node has a left and right child. Either child, or both children, may be missing. Figure 3-2 illustrates a binary search tree. Assuming k represents the value of a given node, then a binary search tree also has the following property: all children to the left of the node have values smaller than k, and all children to the right of the node have values larger than k. The top of a tree is known as the root, and the exposed nodes at the bottom are known as leaves. 
Implementation of Binary Search Tree
# include <stdio.h>
# include <malloc.h>
struct node
{
int info;
struct node *lchild;
struct node *rchild;
}*root;
main()
{
int choice,num;
root=NULL;
clrscr();
while(1)
{
printf("\n");
printf("1.Insert\n");
printf("2.Delete\n");
printf("3.Inorder Traversal\n");
printf("6.Display\n");
printf("7.Quit\n");
printf("Enter your choice : ");
scanf("%d",&choice);
switch(choice)
{
case 1:
printf("Enter the number to be inserted : ");
scanf("%d",&num);
insert(num);
break;
case 2:
printf("Enter the number to be deleted : ");
scanf("%d",&num);
del(num);
break;
case 3:
inorder(root);
break;
case 6:
display(root,1);
break;
case 7:
exit();
default:
printf("Wrong choice\n");
}/*End of switch */
}/*End of while */
}/*End of main()*/
find(int item,struct node **par,struct node **loc)
{
struct node *ptr,*ptrsave;
if(root==NULL) /*tree empty*/
{
*loc=NULL;
*par=NULL;
return;
}
if(item==root->info) /*item is at root*/
{
*loc=root;
*par=NULL;
return;
}
/*Initialize ptr and ptrsave*/
if(item<root->info)
ptr=root->lchild;
else
ptr=root->rchild;
ptrsave=root;
while(ptr!=NULL)
{
if(item==ptr->info)
{ *loc=ptr;
*par=ptrsave;
return;
}
ptrsave=ptr;
if(item<ptr->info)
ptr=ptr->lchild;
else
ptr=ptr->rchild;
}/*End of while */
*loc=NULL; /*item not found*/
*par=ptrsave;
}/*End of find()*/
insert(int item)
{ struct node *tmp,*parent,*location;
find(item,&parent,&location);
if(location!=NULL)
{
printf("Item already present");
return;
}
tmp=(struct node *)malloc(sizeof(struct node));
tmp->info=item;
tmp->lchild=NULL;
tmp->rchild=NULL;
if(parent==NULL)
root=tmp;
else
if(item<parent->info)
parent->lchild=tmp;
else
parent->rchild=tmp;
}/*End of insert()*/
del(int item)
{
struct node *parent,*location;
if(root==NULL)
{
printf("Tree empty");
return;
}
find(item,&parent,&location);
if(location==NULL)
{
printf("Item not present in tree");
return;
}
if(location->lchild==NULL && location->rchild==NULL)
case_a(parent,location);
if(location->lchild!=NULL && location->rchild==NULL)
case_b(parent,location);
if(location->lchild==NULL && location->rchild!=NULL)
case_b(parent,location);
if(location->lchild!=NULL && location->rchild!=NULL)
case_c(parent,location);
free(location);
}/*End of del()*/
case_a(struct node *par,struct node *loc )
{
if(par==NULL) /*item to be deleted is root node*/
root=NULL;
else
if(loc==par->lchild)
par->lchild=NULL;
else
par->rchild=NULL;
}/*End of case_a()*/
case_b(struct node *par,struct node *loc)
{
struct node *child;
/*Initialize child*/
if(loc->lchild!=NULL) /*item to be deleted has lchild */
child=loc->lchild;
else /*item to be deleted has rchild */
child=loc->rchild;
if(par==NULL ) /*Item to be deleted is root node*/
root=child;
else
if( loc==par->lchild) /*item is lchild of its parent*/
par->lchild=child;
else /*item is rchild of its parent*/
par->rchild=child;
}/*End of case_b()*/
case_c(struct node *par,struct node *loc)
{
struct node *ptr,*ptrsave,*suc,*parsuc;
/*Find inorder successor and its parent*/
ptrsave=loc;
ptr=loc->rchild;
while(ptr->lchild!=NULL)
{
ptrsave=ptr;
ptr=ptr->lchild;
}
suc=ptr;
parsuc=ptrsave;
if(suc->lchild==NULL && suc->rchild==NULL)
case_a(parsuc,suc);
else
case_b(parsuc,suc);
if(par==NULL) /*if item to be deleted is root node */
root=suc;
else
if(loc==par->lchild)
par->lchild=suc;
else
par->rchild=suc;
suc->lchild=loc->lchild;
suc->rchild=loc->rchild;
}/*End of case_c()*/
inorder(struct node *ptr)
{
if(root==NULL)
{
printf("Tree is empty");
return;
}
if(ptr!=NULL)
{
inorder(ptr->lchild);
printf("%d ",ptr->info);
inorder(ptr->rchild);
}
}/*End of inorder()*/
display(struct node *ptr,int level)
{
int i;
if ( ptr!=NULL )
{
display(ptr->rchild, level+1);
printf("\n");
for (i = 0; i < level; i++)
printf(" ");
printf("%d", ptr->info);
display(ptr->lchild, level+1);
}/*End of if*/
}/*End of display()*/
