Detect and Remove Loop in Linked List
Last Updated :
23 Jul, 2025
Given the head of a linked list that may contain a loop. A loop means that the last node of the linked list is connected back to a node in the same list. The task is to remove the loop from the linked list (if it exists).
Example:
Input:
Output: 1 -> 3 -> 4
Explanation: The Loop is removed from the above example.
Input:
Output: 1 -> 8 -> 3 -> 4
Explanation: There is no Loop in the above example.
[Naive Approach] Detect and Remove Loop using Hashing - O(n) Time and O(n) Space
The idea is to start traversing the Linked List from head node and while traversing insert each node into the HashSet. Also, maintain a prev pointer which points to the previous node of the current node. If there is a loop present in the Linked List, there will be a node which will be already present in the hash set.
- If there is a node which is already present in the HashSet, then update the next pointer of prev to NULL to remove the loop from the linked list.
- else, if there is no node which is already present in the HashSet , then there is no loop in the linked list.
C++
//Driver Code Starts
// C++ code to detect and remove loop in
// linked list using hashing
#include <bits/stdc++.h>
using namespace std;
struct Node {
int data;
Node *next;
Node(int x) {
data = x;
next = nullptr;
}
};
void printList(Node *curr) {
while (curr != nullptr) {
cout << curr->data << " ";
curr = curr->next;
}
cout << endl;
}
//Driver Code Ends
// Function to detect and remove loop in
// a linked list
void removeLoop(Node *head) {
// hash set to hash addresses of
// the linked list nodes
unordered_set<Node *> st;
// pointer to prev node
Node *prev = nullptr;
while (head != nullptr) {
// if node not present in the map,
// insert it in the map
if (st.find(head) == st.end()) {
st.insert(head);
prev = head;
head = head->next;
}
// if present, it is a cycle, make
// last node's next pointer NULL
else {
prev->next = nullptr;
break;
}
}
}
//Driver Code Starts
int main() {
// Create a hard-coded linked list:
// 1 -> 3 -> 4
Node *head = new Node(1);
head->next = new Node(3);
head->next->next = new Node(4);
// Create a loop
head->next->next->next = head->next;
removeLoop(head);
printList(head);
return 0;
}
//Driver Code Ends
//Driver Code Starts
// Java code to detect and remove loop in linked
// list using hashing
import java.util.HashSet;
class Node {
int data;
Node next;
Node(int x) {
data = x;
next = null;
}
}
class GfG {
static void printList(Node curr) {
while (curr != null) {
System.out.print(curr.data + " ");
curr = curr.next;
}
System.out.println();
}
//Driver Code Ends
// Function to detect and remove loop in a linked list
static void removeLoop(Node head) {
// hash set to hash addresses of
// the linked list nodes
HashSet<Node> st = new HashSet<>();
// pointer to prev node
Node prev = null;
while (head != null) {
// if node not present in the map,
// insert it in the map
if (!st.contains(head)) {
st.add(head);
prev = head;
head = head.next;
}
// if present, it is a cycle, make
// last node's next pointer NULL
else {
prev.next = null;
break;
}
}
}
//Driver Code Starts
public static void main(String[] args) {
// Create a hard-coded linked list:
// 1 -> 3 -> 4
Node head = new Node(1);
head.next = new Node(3);
head.next.next = new Node(4);
// Create a loop
head.next.next.next = head.next;
removeLoop(head);
printList(head);
}
}
//Driver Code Ends
#Driver Code Starts
# Python code to detect and remove loop in linked
# list using hashing
class Node:
def __init__(self, x):
self.data = x
self.next = None
def printList(curr):
# Function to print the linked list
while curr:
print(curr.data, end=' ')
curr = curr.next
print()
#Driver Code Ends
def removeLoop(head):
# Function to detect and remove loop from linked list
nodeSet = set()
prev = None
while head:
# If node is already in the set, remove the loop
if head in nodeSet:
prev.next = None
return
# Add node to the set and move forward
nodeSet.add(head)
prev = head
head = head.next
#Driver Code Starts
if __name__ == "__main__":
# Create a hard-coded linked list:
# 1 -> 3 -> 4
head = Node(1)
head.next = Node(3)
head.next.next = Node(4)
# Create a loop
head.next.next.next = head.next
removeLoop(head)
printList(head)
#Driver Code Ends
//Driver Code Starts
// C# code to detect and remove loop in a
// linked list using hashing
using System;
using System.Collections.Generic;
class Node {
public int data;
public Node next;
public Node(int x) {
data = x;
next = null;
}
}
class GfG {
static void PrintList(Node curr) {
while (curr != null) {
Console.Write(curr.data + " ");
curr = curr.next;
}
}
//Driver Code Ends
// Function to detect and remove loop from linked list
static void removeLoop(Node head) {
HashSet<Node> st = new HashSet<Node>();
Node prev = null;
while (head != null) {
// If we have already seen this node in
// hash set, it means there is a cycle.
// Set the next of the previous pointer
// to null to remove the cycle.
if (st.Contains(head)) {
prev.next = null;
return;
}
// If we are seeing the node for the first time,
// insert it in hash set.
else {
st.Add(head);
prev = head;
head = head.next;
}
}
}
//Driver Code Starts
static void Main(string[] args) {
// Create a hard-coded linked list:
// 1 -> 3 -> 4
Node head = new Node(1);
head.next = new Node(3);
head.next.next = new Node(4);
// Create a loop
head.next.next.next = head.next;
removeLoop(head);
PrintList(head);
}
}
//Driver Code Ends
//Driver Code Starts
// JavaScript code to detect and remove loop in a
// linked list using hashing
class Node {
constructor(x) {
this.data = x;
this.next = null;
}
}
function printList(curr) {
while (curr != null) {
console.log(curr.data + " ");
curr = curr.next;
}
}
//Driver Code Ends
// Function to detect and remove loop from linked list
function removeLoop(head) {
let st = new Set();
let prev = null;
while (head != null) {
// If node is already in the set, remove the loop
if (st.has(head)) {
prev.next = null;
return;
}
// Add node to the set and move forward
st.add(head);
prev = head;
head = head.next;
}
}
//Driver Code Starts
// Create a hard-coded linked list:
// 1 -> 3 -> 4
head = new Node(1);
head.next = new Node(3);
head.next.next = new Node(4);
// Create a loop
head.next.next.next = head.next;
removeLoop(head);
printList(head);
//Driver Code Ends
Time Complexity: O(n), Where n is the number of nodes in the linked list.
Auxiliary Space: O(n), Where n is the number of nodes in the linked list(due to hashing).
[Efficient Approach] Using Floyd's Cycle Detection Algorithm - O(n) Time and O(1) Space
This approach can be divided into two parts:
- Use two pointers, slow and fast and initialize them with the head of the linked list.
- Move the fast pointer forward by two nodes and move the slow pointer forward by one node.
- If the slow and fast pointer points to the same node, loop is found.
- Else if the fast pointer reaches NULL, then no loop is found.
- Else repeat the above steps till we reach the end of the linked list or a loop is found.
2. Remove Loop in Linked List (if any):
The idea is similar to finding the starting node of Loop in a Linked List. For this, we will point the slow pointer to head node and fast pointer will remain at its position. Both slow and fast pointers move one step ahead until fast->next is not equals to slow->next. When slow->next equals to fast->next we can easily point fast->next to NULL to remove the loop.
C++
//Driver Code Starts
// C++ program Using Floyd's Cycle Detection Algorithm
#include <bits/stdc++.h>
using namespace std;
struct Node {
int data;
Node *next;
Node(int x) {
data = x;
next = nullptr;
}
};
void printList(Node *curr) {
while (curr != nullptr) {
cout << curr->data << " ";
curr = curr->next;
}
cout << endl;
}
//Driver Code Ends
// Function to detect and remove loop in a linked list that
// may contain loop
void removeLoop(Node *head) {
// If list is empty or has only one node without loop
if (head == nullptr || head->next ==nullptr)
return;
Node *slow = head, *fast = head;
// Move slow and fast 1 and 2 steps ahead respectively.
slow = slow->next;
fast = fast->next->next;
// Search for loop using slow and fast pointers
while (fast && fast->next) {
if (slow == fast)
break;
slow = slow->next;
fast = fast->next->next;
}
// If loop exists
if (slow == fast) {
slow = head;
// this check is needed when slow and fast both meet
// at the head of the LL
if (slow == fast)
while (fast->next != slow)
fast = fast->next;
else {
while (slow->next != fast->next) {
slow = slow->next;
fast = fast->next;
}
}
// since fast->next is the looping point
fast->next = nullptr;
}
}
//Driver Code Starts
int main() {
// Create a hard-coded linked list:
// 1 -> 3 -> 4
Node *head = new Node(1);
head->next = new Node(3);
head->next->next = new Node(4);
// Create a loop
head->next->next->next = head->next;
removeLoop(head);
printList(head);
return 0;
}
//Driver Code Ends
//Driver Code Starts
// C program Using Floyd's Cycle Detection Algorithm
#include <stdio.h>
#include <stdlib.h>
struct Node {
int key;
struct Node *next;
};
void printList(struct Node *curr) {
while (curr != NULL) {
printf("%d ", curr->key);
curr = curr->next;
}
printf("
");
}
//Driver Code Ends
// Function to detect and remove loop in a linked list that
// may contain loop
void removeLoop(struct Node *head) {
// If list is empty or has only one node without loop
if (head == NULL || head->next == NULL)
return;
struct Node *slow = head, *fast = head;
// Move slow and fast 1 and 2 steps ahead respectively.
slow = slow->next;
fast = fast->next->next;
// Search for loop using slow and fast pointers
while (fast && fast->next) {
if (slow == fast)
break;
slow = slow->next;
fast = fast->next->next;
}
// If loop exists
if (slow == fast) {
slow = head;
// this check is needed when slow and fast both meet
// at the head of the LL
if (slow == fast)
while (fast->next != slow)
fast = fast->next;
else {
while (slow->next != fast->next) {
slow = slow->next;
fast = fast->next;
}
}
// since fast->next is the looping point
// remove loop
fast->next = NULL;
}
}
//Driver Code Starts
struct Node *createNode(int key) {
struct Node *curr =
(struct Node *)malloc(sizeof(struct Node));
curr->key = key;
curr->next = NULL;
return curr;
}
int main() {
// Create a hard-coded linked list:
// 1 -> 3 -> 4
struct Node *head = createNode(1);
head->next = createNode(3);
head->next->next = createNode(4);
// Create a loop
head->next->next->next = head->next;
removeLoop(head);
printList(head);
return 0;
}
//Driver Code Ends
//Driver Code Starts
// Java program Using Floyd's Cycle Detection Algorithm
class Node {
int data;
Node next;
Node(int x) {
data = x;
next = null;
}
}
class GfG {
//Driver Code Ends
// Function that detects loop in the list
static void removeLoop(Node head) {
// If list is empty or has only one node
// without loop
if (head == null || head.next == null)
return;
Node slow = head, fast = head;
// Move slow and fast 1 and 2 steps
// ahead respectively.
slow = slow.next;
fast = fast.next.next;
// Search for loop using slow and fast pointers
while (fast != null && fast.next != null) {
if (slow == fast)
break;
slow = slow.next;
fast = fast.next.next;
}
// If loop exists
if (slow == fast) {
slow = head;
if (slow != fast) {
while (slow.next != fast.next) {
slow = slow.next;
fast = fast.next;
}
// since fast->next is the looping point
// remove loop
fast.next = null;
}
// This case is added if fast and slow
// pointer meet at first position.
else {
while(fast.next != slow) {
fast = fast.next;
}
fast.next = null;
}
}
}
//Driver Code Starts
static void printList(Node curr) {
while (curr != null) {
System.out.print(curr.data + " ");
curr = curr.next;
}
}
public static void main(String[] args) {
// Create a hard-coded linked list:
// 1 -> 3 -> 4
Node head = new Node(1);
head.next = new Node(3);
head.next.next = new Node(4);
// Create a loop
head.next.next.next = head.next;
removeLoop(head);
printList(head);
}
}
//Driver Code Ends
#Driver Code Starts
# Python program Using Floyd's Cycle Detection Algorithm
class Node:
def __init__(self, x):
self.data = x
self.next = None
#Driver Code Ends
# Function to remove the loop from the linked list
def removeLoop(head):
# If the list is empty or has only one node
# without a loop
if head is None or head.next is None:
return
slow = head
fast = head
# Move slow and fast pointers; slow moves 1 step,
# fast moves 2 steps
while slow and fast and fast.next:
slow = slow.next
fast = fast.next.next
# If slow and fast meet, a loop is detected
if slow == fast:
slow = head
# Move slow and fast pointers to find the node
# where the loop starts
while slow != fast:
slow = slow.next
fast = fast.next
# Traverse the loop to find the node where the
# loop ends and remove it
while fast.next != slow:
fast = fast.next
fast.next = None
#Driver Code Starts
def printList(curr):
while curr:
print(curr.data, end=' ')
curr = curr.next
print()
if __name__ == "__main__":
# Create a linked list:
# 1 -> 3 -> 4
head = Node(1)
head.next = Node(3)
head.next.next = Node(4)
# Creating a loop
head.next.next.next = head.next
# Remove the loop from the linked list
removeLoop(head)
printList(head)
#Driver Code Ends
//Driver Code Starts
// C# program Using Floyd's Cycle Detection Algorithm
class Node {
public int data;
public Node next;
public Node(int x) {
data = x;
next = null;
}
}
class GfG {
//Driver Code Ends
// Function that detects loop in the list
static void removeLoop(Node head) {
// If list is empty or has only one node
// without loop
if (head == null || head.next == null)
return;
Node slow = head, fast = head;
// Move slow and fast 1 and 2 steps
// ahead respectively.
slow = slow.next;
fast = fast.next.next;
// Search for loop using slow and fast pointers
while (fast != null && fast.next != null) {
if (slow == fast)
break;
slow = slow.next;
fast = fast.next.next;
}
// If loop exists
if (slow == fast) {
slow = head;
if (slow != fast) {
while (slow.next != fast.next) {
slow = slow.next;
fast = fast.next;
}
// since fast->next is the looping point
// remove loop
fast.next = null;
}
// This case is added if fast and slow pointer
// meet at first position.
else {
while (fast.next != slow) {
fast = fast.next;
}
fast.next = null;
}
}
}
//Driver Code Starts
static void printList(Node curr) {
while (curr != null) {
System.Console.Write(curr.data + " ");
curr = curr.next;
}
}
static void Main(string[] args) {
// Create a hard-coded linked list:
// 1 -> 3 -> 4
Node head = new Node(1);
head.next = new Node(3);
head.next.next = new Node(4);
// Create a loop
head.next.next.next = head.next;
removeLoop(head);
printList(head);
}
}
//Driver Code Ends
//Driver Code Starts
// JavaScript program Using Floyd's Cycle Detection Algorithm
class Node {
constructor(x) {
this.data = x;
this.next = null;
}
}
//Driver Code Ends
// Function to detect and remove loop in the linked list
function removeLoop(head) {
// If list is empty or has only one node without loop
if (head == null || head.next == null) return;
let slow = head, fast = head;
// Move slow and fast 1 and 2 steps
// ahead respectively
slow = slow.next;
fast = fast.next.next;
// Search for loop using slow and fast pointers
while (fast != null && fast.next != null) {
if (slow == fast) break;
slow = slow.next;
fast = fast.next.next;
}
// If loop exists
if (slow == fast) {
slow = head;
// If the loop starts at the head of the list
if (slow != fast) {
while (slow.next != fast.next) {
slow = slow.next;
fast = fast.next;
}
// Remove the loop
fast.next = null;
} else {
// Special case when loop starts at the head
while (fast.next != slow) {
fast = fast.next;
}
fast.next = null;
}
}
}
//Driver Code Starts
function printList(curr) {
let result = [];
while (curr != null) {
result.push(curr.data);
curr = curr.next;
}
console.log(result.join(' '));
}
// Create a hard-coded linked list:
// 1 -> 3 -> 4
let head = new Node(1);
head.next = new Node(3);
head.next.next = new Node(4);
// Create a loop
head.next.next.next = head.next;
removeLoop(head);
printList(head);
//Driver Code Ends
Time Complexity: O(n), where n is the number of nodes in the Linked List
Auxiliary Space: O(1)
For more details about the working & proof of this algorithm, Please refer to this article, How does Floyd’s Algorithm works.
Reverse a linked list in groups of size k in JS
Reverse a linked list in groups of size k in JS
Detect loop
Detect loop using floyd cycle detection
Remove Loop in Linked List
Similar Reads
Basics & Prerequisites
Data Structures
Array Data StructureIn this article, we introduce array, implementation in different popular languages, its basic operations and commonly seen problems / interview questions. An array stores items (in case of C/C++ and Java Primitive Arrays) or their references (in case of Python, JS, Java Non-Primitive) at contiguous
3 min read
String in Data StructureA string is a sequence of characters. The following facts make string an interesting data structure.Small set of elements. Unlike normal array, strings typically have smaller set of items. For example, lowercase English alphabet has only 26 characters. ASCII has only 256 characters.Strings are immut
2 min read
Hashing in Data StructureHashing is a technique used in data structures that efficiently stores and retrieves data in a way that allows for quick access. Hashing involves mapping data to a specific index in a hash table (an array of items) using a hash function. It enables fast retrieval of information based on its key. The
2 min read
Linked List Data StructureA linked list is a fundamental data structure in computer science. It mainly allows efficient insertion and deletion operations compared to arrays. Like arrays, it is also used to implement other data structures like stack, queue and deque. Here’s the comparison of Linked List vs Arrays Linked List:
2 min read
Stack Data StructureA Stack is a linear data structure that follows a particular order in which the operations are performed. The order may be LIFO(Last In First Out) or FILO(First In Last Out). LIFO implies that the element that is inserted last, comes out first and FILO implies that the element that is inserted first
2 min read
Queue Data StructureA Queue Data Structure is a fundamental concept in computer science used for storing and managing data in a specific order. It follows the principle of "First in, First out" (FIFO), where the first element added to the queue is the first one to be removed. It is used as a buffer in computer systems
2 min read
Tree Data StructureTree Data Structure is a non-linear data structure in which a collection of elements known as nodes are connected to each other via edges such that there exists exactly one path between any two nodes. Types of TreeBinary Tree : Every node has at most two childrenTernary Tree : Every node has at most
4 min read
Graph Data StructureGraph Data Structure is a collection of nodes connected by edges. It's used to represent relationships between different entities. If you are looking for topic-wise list of problems on different topics like DFS, BFS, Topological Sort, Shortest Path, etc., please refer to Graph Algorithms. Basics of
3 min read
Trie Data StructureThe Trie data structure is a tree-like structure used for storing a dynamic set of strings. It allows for efficient retrieval and storage of keys, making it highly effective in handling large datasets. Trie supports operations such as insertion, search, deletion of keys, and prefix searches. In this
15+ min read
Algorithms
Searching AlgorithmsSearching algorithms are essential tools in computer science used to locate specific items within a collection of data. In this tutorial, we are mainly going to focus upon searching in an array. When we search an item in an array, there are two most common algorithms used based on the type of input
2 min read
Sorting AlgorithmsA Sorting Algorithm is used to rearrange a given array or list of elements in an order. For example, a given array [10, 20, 5, 2] becomes [2, 5, 10, 20] after sorting in increasing order and becomes [20, 10, 5, 2] after sorting in decreasing order. There exist different sorting algorithms for differ
3 min read
Introduction to RecursionThe process in which a function calls itself directly or indirectly is called recursion and the corresponding function is called a recursive function. A recursive algorithm takes one step toward solution and then recursively call itself to further move. The algorithm stops once we reach the solution
14 min read
Greedy AlgorithmsGreedy algorithms are a class of algorithms that make locally optimal choices at each step with the hope of finding a global optimum solution. At every step of the algorithm, we make a choice that looks the best at the moment. To make the choice, we sometimes sort the array so that we can always get
3 min read
Graph AlgorithmsGraph is a non-linear data structure like tree data structure. The limitation of tree is, it can only represent hierarchical data. For situations where nodes or vertices are randomly connected with each other other, we use Graph. Example situations where we use graph data structure are, a social net
3 min read
Dynamic Programming or DPDynamic Programming is an algorithmic technique with the following properties.It is mainly an optimization over plain recursion. Wherever we see a recursive solution that has repeated calls for the same inputs, we can optimize it using Dynamic Programming. The idea is to simply store the results of
3 min read
Bitwise AlgorithmsBitwise algorithms in Data Structures and Algorithms (DSA) involve manipulating individual bits of binary representations of numbers to perform operations efficiently. These algorithms utilize bitwise operators like AND, OR, XOR, NOT, Left Shift, and Right Shift.BasicsIntroduction to Bitwise Algorit
4 min read
Advanced
Segment TreeSegment Tree is a data structure that allows efficient querying and updating of intervals or segments of an array. It is particularly useful for problems involving range queries, such as finding the sum, minimum, maximum, or any other operation over a specific range of elements in an array. The tree
3 min read
Pattern SearchingPattern searching algorithms are essential tools in computer science and data processing. These algorithms are designed to efficiently find a particular pattern within a larger set of data. Patten SearchingImportant Pattern Searching Algorithms:Naive String Matching : A Simple Algorithm that works i
2 min read
GeometryGeometry is a branch of mathematics that studies the properties, measurements, and relationships of points, lines, angles, surfaces, and solids. From basic lines and angles to complex structures, it helps us understand the world around us.Geometry for Students and BeginnersThis section covers key br
2 min read
Interview Preparation
Practice Problem