Starting Point of Loop

Step0 / 10

PhaseInitialize

Nodes6

Start—

Code

1def detectCycle(head):
2    if not head or not head.next:
3        return None
4 
5    # Phase 1: Detect if cycle exists
6    slow = fast = head
7    while fast and fast.next:
8        slow = slow.next
9        fast = fast.next.next
10        if slow == fast:
11            break
12    else:
13        return None  # No cycle
14 
15    # Phase 2: Find starting point
16    ptr1 = head
17    ptr2 = slow
18    while ptr1 != ptr2:
19        ptr1 = ptr1.next
20        ptr2 = ptr2.next
21 
22    return ptr1

Function definition

Linked List

INIT

Step

0 / 10

Phase

Initialize

Nodes

Progress

Processing

Searching for the starting point of the cycle...

Pointer State

Pointers

slow

fast

ptr1

null

ptr2

null

Key Insight

After slow/fast meet, moving ptr1 to head and advancing both by 1 until they meet finds the cycle start mathematically.

Explanation

Setup

Initialize Pointers

Set both slow and fast pointers to the head of the list.

💡 Tip

Phase 1: Detect if a cycle exists.