What is the Fast & Slow Pointers pattern in coding interviews?

Cycle detection and midpoint traversal using Floyd's Tortoise and Hare algorithm It is categorized under Fundamentals and features 7 curated interview problems across Easy (4), Medium (3), and Hard (0) difficulties.

Which tech companies frequently test Fast & Slow Pointers?

Companies frequently asking Fast & Slow Pointers questions include Google, Amazon, Bloomberg, NeetCode 150, Microsoft, Meta.

How do I master the Fast & Slow Pointers pattern?

Start by studying the algorithmic strategy and template code, then practice Easy and Medium problems before attempting Hard problems under timed conditions.

Fast & Slow Pointers

Fundamentals

Cycle detection and midpoint traversal using Floyd's Tortoise and Hare algorithm

7 problems·4 Easy·3 Medium·0 Hard

Pattern Study Guide & Cheat Sheet· O(n) linear scan · O(1) strictly constant space

▼

Advance two pointers at different speeds (usually 1 step and 2 steps) to detect cycles or locate the middle node in O(N) time and O(1) space.

Core Invariant: In a cycle of length C, each iteration closes the gap between fast and slow by 1. Fast catches slow in <= C steps. Distance from head to cycle entry equals distance from collision point to cycle entry.

Recognize it (Keywords & Signals)

Detect cycle in a Linked List
Find the exact start / entry node of a cycle
Find middle node of a Linked List in a single pass
Find duplicate number in array without modifying it (Floyd cycle)
Happy Number (detect repeating loop in sum of squared digits)

When NOT to use

Need random access or already have O(N) auxiliary space permitted to use a Hash Set.

How to solve (Step-by-step)

1.Initialize `slow = head`, `fast = head`.
2.Advance `slow = slow.next` and `fast = fast.next.next` inside `while fast and fast.next`.
3.If `slow == fast`, a cycle is detected.
4.To find cycle start: reset `slow = head`. Move both `slow` and `fast` 1 step at a time until they meet. The meeting node is the cycle entry.
5.For middle node: when fast reaches the end, slow is at the middle.

Watch for (Interview Traps)

Dereferencing `fast.next.next` without checking both `fast` and `fast.next` (causes AttributeError)
Even-length lists: slow stops at the second middle node if starting both at head
Assuming an array has no cycle without checking bounds when modeling indices as pointers

Floyd's Cycle Detection & Entry Finder

class ListNode:
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next

def detect_cycle_entry(head: ListNode | None) -> ListNode | None:
    slow = fast = head
    
    # Phase 1: Determine if a cycle exists
    while fast and fast.next:
        slow = slow.next
        fast = fast.next.next
        if slow == fast:
            break
    else:
        return None  # Reached end of list -> No cycle
        
    # Phase 2: Find cycle entry node
    slow = head
    while slow != fast:
        slow = slow.next
        fast = fast.next
        
    return slow  # Both pointers meet at cycle entrance

Cost: O(n) linear scan · O(1) strictly constant space (Guarantees O(1) space where a visited hash set would take O(n).)

Canonical problems

#141 Linked List Cycle: Verify if fast pointer collides with slow pointer

#142 Linked List Cycle II: Reset slow to head to find exact entry node

#876 Middle of the Linked List: When fast hits the end, slow is at the midpoint

#287 Find the Duplicate Number: Treat array indices as pointers and find cycle entrance

⌘K

#202Happy Number

Easy·30 companies·Max freq 100%·Acc 59.9%

BlackRockXNike+27

#141Linked List Cycle

Easy·26 companies·Max freq 100%·Acc 54.7%

YahooNeetCode 150NeetCode 150+23

#287Find the Duplicate Number

Medium·24 companies·Max freq 90%·Acc 64.6%

NianticNeetCode 150NeetCode 150+21

#234Palindrome Linked List

Easy·18 companies·Max freq 91%·Acc 58.3%

IXLAMDIntel+15

#876Middle of the Linked List

Easy·13 companies·Max freq 88%·Acc 82.1%

Walmart LabsAMDAccenture+10

#142Linked List Cycle II

Medium·13 companies·Max freq 77%·Acc 58.5%

PaytmVMwareAmerican Express+10

#457Circular Array Loop

Medium·4 companies·Max freq 38%·Acc 37.8%

Goldman SachsAmazonBloomberg+1

#	Problem	Difficulty	Top Companies↓	Frequency	Acceptance
#202	Happy Number Hash TableMathTwo PointersFloyd's Cycle Finding Algorithm	Easy	BlackRock X Nike Jump Trading+26	100%	59.9%
#141	Linked List Cycle Hash TableLinked ListTwo PointersFloyd's Cycle Finding Algorithm	Easy	Yahoo NeetCode 150 Qualcomm Nagarro+22	100%	54.7%
#287	Find the Duplicate Number ArrayTwo PointersBinary SearchBit ManipulationPigeonhole PrincipleFloyd's Cycle Finding Algorithm	Medium	Niantic NeetCode 150 DP world Nike+20	90%	64.6%
#234	Palindrome Linked List Linked ListTwo PointersStackRecursion	Easy	IXL AMD Intel Amazon+14	91%	58.3%
#876	Middle of the Linked List Linked ListTwo Pointers	Easy	Walmart Labs AMD Accenture Qualcomm+9	88%	82.1%
#142	Linked List Cycle II Hash TableLinked ListTwo PointersFloyd's Cycle Finding Algorithm	Medium	Paytm VMware American Express ByteDance+9	77%	58.5%
#457	Circular Array Loop ArrayHash TableTwo PointersFloyd's Cycle Finding Algorithm	Medium	Goldman Sachs Amazon Bloomberg Google	38%	37.8%

Showing 7 of 7 problems in Fast & Slow PointersFiltered: All Companies