Question

I’m trying to implement a program for finding a starting node of circular linked list. My code is-

struct node
{
char data;
struct node *link;
} ;

char FindStartNode(struct node **q)
{
    struct node *r,*t;
 r = *q; 
 t = *q;
 while(t->link != NULL)
 {
     r = r->link;
     t = t->link->link;
     if(r == t)
     {
         break;
     }
 }
 if(t == NULL )
     return NULL;
 r = *q;
 while(r != t)
 {
     t = t->link;
     r = r->link;
 }
 return t->data;
}


int main()
{
struct node *p;
p = NULL;
char num;
Append(&p,'A');
Append(&p,'B');
Append(&p,'C');
Append(&p,'D');
Append(&p,'E');
Append(&p,'C');
Display(p);
 num = FindStartNode(&p);
printf("\nStarting node of the cycle linked list is:- %c",num);
_getch();
return 0;
}


int Append(struct node **q, char data)
{
struct node *r,*t;
r = (struct node *)malloc(sizeof(struct node));
r->data = data;
r->link = NULL;
if(*q == NULL)
    *q = r;
else
{
  t = *q;
  while(t->link != NULL)
  {
      t = t->link;
  }
  t->link = r;
    }
return 0;
}
int Display(struct node *q)
{
while(q != NULL)
{
    printf("%c\t",q->data);
    q = q->link;
}
return 0;
}

ths is my code. I’m not getting any value in return t->data part or I’m unable to find the start node of cycle ink list.Any help?

Answer 1

 t = t->link->link; // t->link can be null 
   //so t = t->link->link can be a crash or illegal reference

Change the loop to:

 while(t != NULL)
 {
     r = r->link;
     t = t->link;
     if(t == NULL)
       break; // or return no circle
     else t = t->link;
     if(r == t)
     {
         break;
     }
 }

I have gone through your code. Comparing with the algorithm discussion here it seems to be OK. But you are returning a char why dont you return a pointer so that you can check if it is NULL or not. In case it is not null then issue pt->tada. This makes more sense.

I checked you code it seems you are not implementing circular linked list correctly in Append(). I am providing you with a working implementation below. See how I modified Append()

#include <stdio.h>
#include <stdlib.h>

struct node
{
char data;
struct node *link;
} ;

char FindStartNode(struct node **q)
{
    struct node *r,*t;
 r = *q;
 t = *q;
 while(t->link != NULL)
 {
     r = r->link;
     t = t->link->link;
     if(r == t)
     {
         break;
     }
 }
 if(t == NULL )
     return NULL;
 r = *q;
 while(r != t)
 {
     t = t->link;
     r = r->link;
 }
 return t->data;
}

int Append(struct node **q, char data);
int main()
{
struct node *p;
p = NULL;
char num;
Append(&p,'A');
Append(&p,'B');
Append(&p,'C');
Append(&p,'D');
Append(&p,'E');
Append(&p,'C');
//Display(p);
 num = FindStartNode(&p);
printf("\nStarting node of the cycle linked list is:- %c\n",num);
//_getch();
return 0;
}

int Append(struct node **q, char data)
{

struct node *r,*t, *startOfcycle=NULL;
r = (struct node *)malloc(sizeof(struct node));
r->data = data;


r->link = NULL;
if(*q == NULL)
    *q = r;
else
{
  t = *q;
  while(t->link != NULL)
  {
      if(t->data == data)
         startOfcycle = t;

      t = t->link;
  }


  if(startOfcycle == NULL)
  t->link = r;
  else {// there is a cycle point to the start of cycle
     t->link = startOfcycle;
     free(r);
  }
    }
return 0;
}
int Display(struct node *q)
{
while(q != NULL)
{
    printf("%c\t",q->data);
    q = q->link;
}

Please note that Display function is also wrong as runs an infinite loop of the linked list is circular. I have not modified it since it is not relevant to you question. Thanks.