# Cycle Detection in a Singly Linked List

Posted: 10 Dec, 2019
Difficulty: Moderate

## PROBLEM STATEMENT

#### Note: Since, it is binary problem, there is no partial marking. Marks will only be awarded if you get all the test cases correct.

##### Input format :
``````The first line of each test case contains the elements of the singly linked list separated by a single space and terminated by -1 and hence -1 would never be a list element.

The second line contains the integer position "pos" which represents the position (0-indexed) in the linked list where tail connects to. If "pos" is -1, then there is no cycle in the linked list.
``````
##### Output format :
``````The only line of output contains 'true' if linked list has a cycle or 'false' otherwise.

You don't have to explicitly print by yourself. It has been taken care of.
``````
##### Constraints :
``````0 <= N <= 10^6
-1 <= pos < N
-10^9 <= data <= 10^9 and data != -1

Where 'N' is the size of the singly linked list, "pos" represents the position (0-indexed) in the linked list where tail connects to and "data" is the Integer data of singly linked list.

Time Limit: 1 sec
``````
##### Note :
``````Try to solve this problem in O(N) Time Complexity and O(1) space Complexity
`````` Approach 1

We are going to have two loops outer-loop and inner-loop

1. Maintain a count of the number of nodes visited in outer-loop.
2. For every node of the outer-loop, start the inner loop from head.
3. If the inner-loop visits the node next to the outer-loop node, then return true, else repeat the process for the next iteration of outer-loop.
4. If outer-loop reaches the end of list or null, then return false.