How to detect loop in a linked list in java with example

If you want to practice data structure and algorithm programs, you can go through data structure and algorithm interview questions.

One of the most popular interview question nowadays is “How to detect loop/cycle in LinkedList”. So I thought I should cover this question. This question is more related to data structure. You can also find start node of loop if loop exists.

Java Linked List Interview Programs:

First approach that you may think may something look like:

  • Traverse through each node till end , tracking visited node using visited flag.
  • If you find node that is already visited, then there is a loop in LinkedList and if you reach till end while traversing then there is no loop in LinkedList

But problem with above approach is, in most cases you can not change data structure of LinkedList node, so you won’t be able to add visited flag to it.

Efficient approach:

Efficient approach for this problem would be Floyd’s cycle detection algorithm,so steps for this algo would be:
  • Use two pointer fastPtr and slowPtr and initialize both to head of linkedlist
  • Move fastPtr by two nodes and slowPtr by one node in each iteration.
  • If fastPtr and slowPtr meet at some iteration , then there is a loop in linkedlist.
  • If fastPtr reaches to the end of linkedlist without meeting slow pointer then there is no loop in linkedlist (i.e fastPtr->next or fastPtr->next->next become null)

Java code for this algo will be
Above solution work with o(n) time complexity and o(1) space complexity.

Lets create linked list without loop :

Lets create a java program called “LinkedList.java”
Logically our linkedlist can be represented as :

Run above program, you will get following output:

Lets create a loop in linkedlist

Now we will connect last node to the nodewith value 7 and it will create loop in linked list, so change above main method to this:

Logically our linkedlist with loop can be represented as :

Run above program, you will get following output:

so this is how you can detect a loop in linkedlist.
Please go through java interview programs for more such programs.

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Comments

  1. This will looks like wrong for couple of scenarios
    This will work for Odd loops , If the loop exist like this?

    5->6->7>8>1>5>6

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