To determine the ring in a linked list

1. The first type of implementation

BOOL List_is_loop (Slist *head)

{

Slist *slow=head;

Slist *fast=head;

while (Null!=fast && null!=fast->next)

{

slow=slow->next;

fast=fast->next->next;

if (slow==fast) break;

}

　　

No ring: Fast->next is always null when the number of lists is cardinality, and fast is always null when the number of linked lists is even;

Return! (Fast==null | | fast->next==null);

}

2. The second type of implementation

BOOL List_is_loop (Slist *head)

{

Slist *slow=head;

Slist *fast=head;

if (Null==slow | | Null==slow->next)

return false;

　　

Do

{

slow=slow->next;

fast=fast->next->next;

}while (null!=fast && null!=fast->next && slow!=fast)

if (slow==fast)

{

return true;

}

Else

{

return false;

}

}

3. if the link list exists, locate the ring entry point

Suppose slow walked the S-step, then fast took 2s steps (the step number of fast is also equal to S plus the N-turns on the ring), the ring length is R, then:
2s=s+nr
S=nr
Set the entire chain table length L, the inlet ring and the meeting point distance is X, the starting point to the ring entry point distance is a, then
A+x=s=nr
a+x= (n-1) r+r= (n-1) r+l-a
A= (n-1) r+ (l-a-x)
(l-a-x) is the distance from the point of encounter to the ring entry point. So, from the chain head to the ring entry point equals (n-1) loop inner ring + meet point to ring entry point. So you can set a pointer from the link table head and meeting point, each step, two pointers must meet, and meet the 1th is the ring entry point.

1---2---3---4--5--6--7
| |

-----------

As shown: In Node 5 encounter, 1 to 4 distance of a;4 to 5 is x; ring length is R;

slist* List_is_loop_enter (slist *head)

{

Slist *slow=head;

Slist *fast=head;

while (Null!=fast && null!=fast->next)

{

slow=slow->next;

fast=fast->next->next;

if (slow==fast) break;

}

　　

No ring: Fast->next is always null when the number of lists is cardinality, and fast is always null when the number of linked lists is even;

if (Fast==null | | fast->next==null)

return NULL;

　　

Slow=head;

while (Slow!=fast)

{

slow=slow->next;

fast=fast->next;

}

return slow;

}

4. Calculate the ring length

A loop can be performed from the ring inlet.

int List_is_loop_length (slist *head)

{

int length=0;

Slist *slow=head;

Slist *fast=head;

while (Null!=fast && null!=fast->next)

{

slow=slow->next;

fast=fast->next->next;

if (slow==fast) break;

}

　　

No ring: Fast->next is always null when the number of lists is cardinality, and fast is always null when the number of linked lists is even;

if (Fast==null | | fast->next==null)

return 0;

　　

Slow=head;

while (Slow!=fast)

{

slow=slow->next;

fast=fast->next;

}

Start the loop from the ring inlet

slist* P=slow;

Do

{

p=p->next;

length++;

}while (P!=slow)

return length;

}

To determine the ring in a linked list