diff --git a/Recursion programs/Divisibility by 11 and 9.c b/Recursion programs/Divisibility by 11 and 9.c
index 3ac77f1..db985ef 100644
--- a/Recursion programs/Divisibility by 11 and 9.c	
+++ b/Recursion programs/Divisibility by 11 and 9.c	
@@ -1,21 +1,22 @@
 /*Program that tests whether a number is divisible by 11 and 9 or not */
 
 #include<stdio.h>
+#include<stdlib.h>
 int divisibleBy9(long int x);
 int divisibleBy11(long int x);
 
-main( )
+int main( )
 {
 	long int num;
 	printf("Enter the number to be tested : ");
 	scanf("%ld", &num);
 	
-	if(divisibleBy9(num))
+	if((num == 0) || divisibleBy9(labs(num)))
 		printf("The number is divisible by 9\n");
 	else
 		printf("The number is not divisible by 9\n");
 	
-	if(divisibleBy11(num))
+	if(divisibleBy11(labs(num)))
 		printf("The number is divisible by 11\n");
 	else
 		printf("The number is not divisible by 11\n");
@@ -34,12 +35,12 @@ int divisibleBy9( long int n )
 		sumofDigits += n%10;	
 		n/=10;
 	}
-	divisibleBy9(sumofDigits);
+	return divisibleBy9(sumofDigits);
 }/*End of divisibleBy9()*/
 
 int divisibleBy11( long int n )
 {
-	int s1=0, s2=0,diff;
+	int s1=0, s2=0;
 	
 	if(n == 0)
 		return 1;
@@ -53,7 +54,7 @@ int divisibleBy11( long int n )
 		s2 += n%10;
 		n /= 10;
 	}
-	diff = s1>s2 ? (s1-s2) : (s2-s1); 
-	divisibleBy11(diff);
+	return divisibleBy11(labs(s1-s2));
 }/*End of divisibleBy11()*/
 
+
diff --git a/Recursion programs/Even Sum.C b/Recursion programs/Even Sum.C
index badc4eb..fcb5210 100644
--- a/Recursion programs/Even Sum.C	
+++ b/Recursion programs/Even Sum.C	
@@ -1,10 +1,10 @@
 /* Sum of all even numbers in an array */
 #include<stdio.h>
 int sumEven(int arr[], int size);
-main( )
+int main( )
 {
 	int arr[6]={1,2,3,4,8,10};
-	printf("%d\n",sumEven(arr,6));
+	printf("%d\n",sumEven(arr,sizeof(arr)/sizeof(int)));
 }
 int sumEven(int arr[], int size)
 {
diff --git a/Recursion programs/Factorial.c b/Recursion programs/Factorial.c
index 4ff1776..14e684a 100644
--- a/Recursion programs/Factorial.c	
+++ b/Recursion programs/Factorial.c	
@@ -1,23 +1,23 @@
-/*Program to find the factorial of a number by recursive method*/
+/*Program to find the factorial of a number by recursive and iterative methods*/
 
 #include<stdio.h>
+#include<stdlib.h>
 long int fact(int n);
 long int Ifact(int n);
 
-main( )
+int main( )
 {
 	int num;
 	printf("Enter a number : ");
 	scanf("%d", &num);
-	if(num<0)
+	if(num<0) {
 		printf("No factorial for negative number\n");
-	else
-		printf("Factorial of %d is %ld\n", num, fact(num) );
+		exit(1);
+	}
 
-	if(num<0)
-		printf("No factorial for negative number\n");
-	else
-		printf("Factorial of %d is %ld\n", num, Ifact(num) );
+	printf("Factorial of %d is %ld\n", num, fact(num) );
+
+	printf("Factorial of %d is %ld\n", num, Ifact(num) );
 }/*End of main()*/
 
 /*Recursive*/
@@ -39,4 +39,3 @@ long int Ifact(int n)
 	}
 	return fact;
 }/*End of ifact()*/
-
diff --git a/Recursion programs/Input and add n numbers.c b/Recursion programs/Input and add n numbers.c
index e58b548..670171a 100644
--- a/Recursion programs/Input and add n numbers.c	
+++ b/Recursion programs/Input and add n numbers.c	
@@ -1,21 +1,26 @@
 /* Input and add n numbers*/
 #include<stdio.h>
+#include<stdlib.h>
 int	InputAndAdd(int n);
-main()
+int main()
 {
 	int n;
 	printf("Enter n :");
 	scanf("%d",&n);
+	if (n < 1)
+	{
+		printf("Input must be a positive integer\n");
+		exit(1);
+	}
 	printf("%d\n",InputAndAdd(n));
 }
 int	InputAndAdd(int n)
 {
 	int a;
 	printf("Enter a number : ");      
-    scanf("%d",&a);
+	scanf("%d",&a);
 	if (n == 1)
 	    return a;
 	else
 	    return a + InputAndAdd(n-1);
 }
-
diff --git a/Recursion programs/Number in Words.C b/Recursion programs/Number in Words.C
index b0c3716..96d690c 100644
--- a/Recursion programs/Number in Words.C	
+++ b/Recursion programs/Number in Words.C	
@@ -3,6 +3,8 @@
 #include<stdio.h>
 void f(int n); 
 
+const char* words[] = {"zero", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine"};
+
 int main() 
 { 
 	int num = 12340;
@@ -16,17 +18,5 @@ void f(int n)
 
 	f(n/10);
 
-	switch(n%10)
-	{
-		case 0: printf("zero ");break;
-		case 1: printf("one ");break;
-		case 2: printf("two ");break;
-		case 3: printf("three ");break;
-		case 4: printf("four ");break;
-		case 5: printf("five ");break;
-		case 6: printf("six ");break;
-		case 7: printf("seven ");break;
-		case 8: printf("eight ");break;
-		case 9: printf("nine ");break;
-	}
+	printf("%s ", words[n%10]);
 } 
diff --git a/Recursion programs/Reverse of text line.C b/Recursion programs/Reverse of text line.C
index c92aeaa..1bc52ac 100644
--- a/Recursion programs/Reverse of text line.C	
+++ b/Recursion programs/Reverse of text line.C	
@@ -1,16 +1,31 @@
 /* Enter a line of text and Reverse it*/
 #include<stdio.h>
+#include<stdlib.h>
+
 void func(void);
-main( )
+
+int main( )
 {
 	printf("Enter text :\n");
 	func();
 	printf("\n");
 }/*End of main()*/
+
 void func(void)
 {
-	char c;
-	if((c=getchar())!='\n')
+	int c;
+
+	c = getchar();
+	switch (c)
+	{
+	case EOF:
+	case '\n':
+		break;
+	default:
 		func();
-	putchar(c);
+		break;
+	}
+
+	if (c != EOF)
+		putchar(c);
 }
diff --git a/Sorting/Sort Bubble.c b/Sorting/Sort Bubble.c
index 7b4a1eb..37f70e9 100644
--- a/Sorting/Sort Bubble.c	
+++ b/Sorting/Sort Bubble.c	
@@ -1,15 +1,22 @@
 /*Program of sorting using bubble sort*/
 
 #include <stdio.h>
+#include <stdlib.h>
 #define MAX 100
 
-main()
+int main()
 {
 	int arr[MAX],i,j,temp,n,xchanges;
 
 	printf("Enter the number of elements : ");
 	scanf("%d",&n);
 
+	if(n > MAX)
+	{
+		printf("number of elements must not exceed %d\n", MAX);
+		exit(1);
+	}
+
 	for(i=0; i<n; i++)
 	{
 		printf("Enter element %d : ",i+1);
@@ -39,4 +46,3 @@ main()
 		printf("%d ",arr[i]);
 	printf("\n");
 }/*End of main()*/
-
diff --git a/Stacks and Queues/Stack using Array.c b/Stacks and Queues/Stack using Array.c
index 9a84e2c..f38e9f1 100644
--- a/Stacks and Queues/Stack using Array.c	
+++ b/Stacks and Queues/Stack using Array.c	
@@ -44,7 +44,7 @@ main()
 			display();
 			break;
 		 case 5:
-			exit(1);
+			exit(0);
 		 default:
 			printf("Wrong choice\n");
 		}/*End of switch*/
diff --git a/Stacks and Queues/Stack using Linked List.c b/Stacks and Queues/Stack using Linked List.c
index fc79800..6986c89 100644
--- a/Stacks and Queues/Stack using Linked List.c	
+++ b/Stacks and Queues/Stack using Linked List.c	
@@ -46,7 +46,7 @@ main()
 			display();
 			break;
 		case 5:
-			exit(1);
+			exit(0);
 		default :
 			printf("Wrong choice\n");
 		}/*End of switch */
diff --git a/Trees/IsBST.c b/Trees/IsBST.c
index d60aba8..b0ddc7e 100644
--- a/Trees/IsBST.c
+++ b/Trees/IsBST.c
@@ -1,7 +1,10 @@
-/* Find whether a binary tree is binary search tree or not*/
+/* Find out whether a binary tree is a binary search tree or not*/
 #include<stdio.h>
 #include<stdlib.h>
+#include<string.h>
 #include<limits.h>
+#include<unistd.h>
+#define SLEN 5
 
 struct node
 {
@@ -15,31 +18,44 @@ void inorder(struct node *ptr);
 struct node *Node(int item);
 int IsBST(struct node *ptr, int MIN, int MAX);
 
-main( )
+int main( )
 {
-	struct node *root1, *root2;
-	
+	struct node *root1, *root2, *root3;
+	char str[SLEN];
+
+	srandom(getpid());
+
+	/* BST */
 	root1 = Node(32);
 	root1->lchild = Node(23);
 	root1->rchild = Node(36);
 	root1->lchild->rchild = Node(25);
 	root1->rchild->lchild = Node(33);
 
+	/* not a BST */
 	root2 = Node(42);
 	root2->lchild = Node(60);
 	root2->rchild = Node(19);
 	root2->lchild->rchild = Node(36);
 	root2->rchild->lchild = Node(41);
 
+	/* random test data */
+	root3 = Node(random() % INT_MAX);
+	root3->lchild = Node(random() % INT_MAX);
+	root3->rchild = Node(random() % INT_MAX);
+	root3->lchild->rchild = Node(random() % INT_MAX);
+	root3->rchild->lchild = Node(random() % INT_MAX);
+
 	display(root1,1);
 	printf("\n\n");
 	inorder(root1);
 	printf("\n\n");
 
-	if( IsBST(root1,INT_MIN,INT_MAX) )  
-		printf("Tree 1 is a BST\n");
+	if( IsBST(root1,INT_MIN,INT_MAX) )
+		str[0] = '\0';  
 	else
-		printf("Tree 1 is not a BST\n");
+		strncpy(str, "not ", SLEN);
+	printf("Tree 1 is %sa BST\n", str);
 
 	display(root2,1);
 	printf("\n\n");
@@ -47,9 +63,21 @@ main( )
 	printf("\n\n");
 
 	if( IsBST(root2,INT_MIN,INT_MAX) )  
-		printf("Tree 2 is a BST\n");
+		str[0] = '\0';  
 	else
-		printf("Tree 2 is not a BST\n");
+		strncpy(str, "not ", SLEN);
+	printf("Tree 2 is %sa BST\n", str);
+
+	display(root3,1);
+	printf("\n\n");
+	inorder(root3);
+	printf("\n\n");
+
+	if( IsBST(root3,INT_MIN,INT_MAX) )  
+		str[0] = '\0';  
+	else
+		strncpy(str, "not ", SLEN);
+	printf("Tree 3 is %sa BST\n", str);
 }/*End of main( )*/
 
 int IsBST(struct node *ptr, int MIN, int MAX)  
@@ -64,6 +92,10 @@ int IsBST(struct node *ptr, int MIN, int MAX)
 struct node *Node(int item)
 {  
 	struct node* tmp = (struct node *)malloc(sizeof(struct node));  
+	if (tmp == NULL) {
+		perror("malloc");
+		exit(1);
+	}
 	tmp->info = item;  
 	tmp->lchild = tmp->rchild = NULL;  
 	return tmp;