#41 Easy Time O(n) Space O(1) Animated

Linked List Cycle

Detect if a linked list has a cycle using Floyd's algorithm.

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Tutor Mode Build the mental model before you memorize the code.

1 Predict
2 Trace
3 Explain
4 Implement

Pattern Pointer rewiring with saved next state

Before changing a link, save the only pointer that would otherwise be lost.

Approach: Floyd's Tortoise and Hare

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Core Insight

Before changing a link, save the only pointer that would otherwise be lost.

Common Pitfall

Always store next before redirecting current.next.

Description

Detect if a linked list has a cycle using Floyd's algorithm.

Constraints

Time Complexity: O(n)
Space Complexity: O(1)

Solution — Floyd's Tortoise and Hare

TimeO(n) SpaceO(1)

Implementation Plan

Translate the invariant into these small, testable moves.

Initialize the pointers.
Save the next node.
Rewire the current node.
Advance both pointers until the list is consumed.

def hasCycle(head):
    slow, fast = head, head
    
    while fast and fast.next:
        slow = slow.next
        fast = fast.next.next
        if slow == fast:
            return True
            
    return False

Add Two Numbers

Find the Duplicate Number

Linked List Cycle

Steps

Description

Constraints

Tags

Solution — Floyd's Tortoise and Hare