Write a function *detectAndCountLoop()* that checks whether a given Linked List contains loop and if loop is present then returns count of nodes in loop. For example, loop is present in below linked list and length of loop is 4. If loop is not present, then function should return 0.

We know that Floyd’s Cycle detection algorithm terminates when fast and slow pointers meet at a common point. We also know that this common point is one of the loop nodes (2 or 3 or 4 or 5 in the above diagram). We store the address of this in a pointer variable say ptr2. Then we start from the head of the Linked List and check for nodes one by one if they are reachable from ptr2. When we find a node that is reachable, we know that this node is the starting node of the loop in Linked List and we can get pointer to the previous of this node.

// C program to count number of nodes // in loop in a linked list if loop is // present #include<stdio.h> #include<stdlib.h> /* Link list node */ struct Node { int data; struct Node* next; }; // Returns count of nodes present in loop. int countNodes(struct Node *n) { int res = 1; struct Node *temp = n; while (temp->next != n) { res++; temp = temp->next; } return res; } /* This function detects and counts loop nodes in the list. If loop is not there in then returns 0 */ int countNodesinLoop(struct Node *list) { struct Node *slow_p = list, *fast_p = list; while (slow_p && fast_p && fast_p->next) { slow_p = slow_p->next; fast_p = fast_p->next->next; /* If slow_p and fast_p meet at some point then there is a loop */ if (slow_p == fast_p) return countNodes(slow_p); } /* Return 0 to indeciate that ther is no loop*/ return 0; } struct Node *newNode(int key) { struct Node *temp = (struct Node*)malloc(sizeof(struct Node)); temp->data = key; temp->next = NULL; return temp; } /* Drier program to test above function*/ int main() { struct Node *head = newNode(1); head->next = newNode(2); head->next->next = newNode(3); head->next->next->next = newNode(4); head->next->next->next->next = newNode(5); /* Create a loop for testing */ head->next->next->next->next->next = head->next; printf("%d \n", countNodesinLoop(head)); return 0; }

Output :

4

**Related Articles : **

Detect loop in a linked list

Detect and Remove Loop in a Linked List

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