DSA interview question

Length of the Loop in a Linked List

Medium · Data structures · Topics: linkedlist, two pointer

The problem

After finding a cycle, the scheduler team wants to know how many tasks are trapped in it, to estimate how much work was repeated. Once Floyd's algorithm finds a meeting point, that point is guaranteed to be inside the cycle. From there, walking forward until you return to the same node counts the cycle's length exactly. If there's no cycle, the length is 0.

Example: 1 -> 2 -> 3 -> 4 -> 5, with 5 linking back to 2 → 4 (the cycle 2, 3, 4, 5).

Function signature

int loopLength(Node* head);  // number of nodes in the cycle, or 0 if there is none

Test cases

Input (values, tail links to index)	Output	Explanation
`[1, 2, 3, 4, 5]`, pos 1	4	Nodes 2, 3, 4, 5
`[25, 14, 19, 33, 10, 21, 39, 90, 58, 45]`, pos 3	7	From 33 to 45
`[1]`, pos 0	1	A self-loop has length 1
`[1, 2]`, pos -1	0	No cycle
`[1, 2, 3]`, pos 0	3	The whole list

(Confirm: return the number of distinct nodes in the cycle; 0 when the list is acyclic.)

How interviewers phrase it

"Find the length of the loop in a linked list" (GeeksforGeeks).
Often asked together with detecting the loop and finding its start.

Clarifying questions to ask

What should be returned when there's no loop? (0.)
May I modify the list?
Is O(1) extra space required?

Follow-ups to expect

"Why is the meeting point inside the cycle?"
"How do you count the length?"
"Can you get the length from the cycle's entrance instead?"
"How does the length help remove the cycle?"
"Complexity?"

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