🚀 https://techinterviews. Below is the implementation of the above approach: Get Unlimited Access to Articles Placeholder text :) In technical interviews, you will be expected to code up an efficient algorithm, talk comfortably about the design, analysis and tradeoffs of a specific algorithm. Internally, pos is used to denote the index of the node that tail's next pointer is Aug 21, 2021 · In a singly linked list, the slow and fast pointers are sufficient to find the middle element, whereas in a doubly linked list, you can adjust the movement of the fast pointer accordingly. Feb 8, 2022 · How can I find a middle element in a linked list by traversing the entire list only once? The length of the list is not given, and I am allowed to only use two pointers. Next, make Enjoy the videos and music you love, upload original content, and share it all with friends, family, and the world on YouTube. Then take one of the two pointers to the head node, the other pointer stay. For the problem of detecting cycles in a linked list, there is a universal solution—the fast and slow pointer method (Floyd’s Cycle Detection Algorithm). Therefore if two pointers start from head and the other from X, both reaches E at the same time, because slow and fast met at -D. Struggling to understand cycle detection in a linked list?This video breaks down LeetCode Problem 141 step by step using the Fast & Slow Pointer (Floyd’s Cyc 4 days ago · It uses two pointers moving at different speeds: a "slow" pointer (tortoise) that advances 1 step at a time and a "fast" pointer (hare) that advances 2 steps at a time. next 2. Sep 4, 2024 · The idea is to first move the fast pointer N steps ahead, then move both fast and slow pointers together until fast reaches the end. Instantiate two pointers where slow points to the head of the list and fast points to head. K rotations Pattern: Fast and slow pointers Understanding the fast and slow pointer pattern Identifying the fast and slow pointer pattern Middle node search Split list in half newEqual halves Dec 2, 2013 · Logic will remain the same of keeping two pointers, one fast and other slow. Floyd’s Cycle Detection Algorithm provides an efficient way to detect loops using constant space. If the fast pointer reaches the end of the list (i. Move one forward by 1 node and the other by 2 nodes. Perfect for coding interview preparation. Learn the two pointers pattern with step-by-step examples, code templates, and LeetCode practice problems. As the name suggests, one pointer (the “fast” pointer) moves faster than the other (the “slow” pointer). The other pointer, the fast pointer, remains at the point where the two pointers initially met. There is a cycle in a linked list if there is some node in the list that can be reached again by continuously following the next pointer (see image below). Shift head node and E until they meet 4 days ago · This page documents the utilities for iterating over kernel linked lists, a fundamental operation in crash-python. Mar 24, 2022 · Enjoy the videos and music you love, upload original content, and share it all with friends, family, and the world on YouTube. The Fast pointer again catches the slow pointer at some time therefore a loop exists in the Dec 9, 2025 · Master the tortoise and hare algorithm with comprehensive templates, mathematical proofs, and real-world applications. It is often used to detect cycles in a linked list. Note that View products Introduction Day 7/90 |⚡ 🚀 Fast & Slow Pointers Explained | Crack Linked List Problems in O (n) | DSA Pattern #2 CTO Bhaiya 10. 11. The kernel extensively uses intrusive doubly-linked lists (`struct listhead`) embedde grokking-the-coding-interview ├───0-1-knapsack ├───bitwise-XOR ├───codesetup ├───cyclic-sort ├───fast-and-slow-pointers ├───in-place-reversal-of-linked-list ├───k-way-merge ├───merge-intervals ├───miscellaneous ├───modified-binary-search ├───sliding-window LeetCode 141 – Linked List Cycle Detection using Fast & Slow pointers. After this card, you will: Understand the structure of singly linked list and doubly linked list; Implement traversal, insertion, deletion in a singly or doubly linked list; Analyze the complexity of different operations in a singly or doubly linked list; Use two-pointer technique (fast-pointer-slow-pointer technique) in the linked list; Solve For the problem of detecting cycles in a linked list, there is a universal solution—the fast and slow pointer method (Floyd’s Cycle Detection Algorithm). It is based on the learning that whenever we move a pointer with twice the speed of another pointer if there exists any cycle, both will meet somewhere in the cycle. How can this be done? Can you solve this real interview question? Linked List Cycle - Given head, the head of a linked list, determine if the linked list has a cycle in it. It is used to efficiently solve several problems by using two pointers. Internally, pos is used to denote the index of the node that tail's next pointer is connected to. Full explanation in the main video 👇#Shorts #DSA #LeetCode #LinkedList #Coding #Progra Understanding Linked Lists - Free download as PDF File (. fast = fast -> next -> next. Iterate over the LL (while slow != fast) 3. The algorithm is to start two pointers slow and fast from the head of the linked list. 1. At any point, if the fast and the slow pointers point to the same node, return True as a loop has been detected. 3. The slow pointer will then be just before the node to be removed, allowing to update the next pointer to skip the target node. Dec 18, 2023 · Using Floyd’s Tortoise & Hare Algorithm, the concept of the slow and fast pointer is used to detect cycles in a linked list. A “slow” pointer advances one node simultaneously, while the “fast” pointer goes two nodes at a time. Here is explained 6 problems related to it. Oct 23, 2019 · By moving at different speeds, the algorithm proves that the two pointers are going to meet eventually. Jul 11, 2025 · We have discussed Floyd's fast and slow pointer algorithms in Detect loop in a linked list. 🎯 Welcome to the Fast and Slow Pointer Pattern Playlist — your essential guide to one of the most powerful and elegant techniques for solving linked list and array problems in coding interviews. Fast/Slow Pointer Strategy There is a tricky technique that uses two pointers to traverse the list. Problem: Linked List Cycle LeetCode 141 - Linked List Cycle [easy] Given a linked list, determine if it has a cycle in it. Apr 9, 2024 · In Python programming, particularly when manipulating linked lists or arrays, mastering the “slow and fast pointer” technique can… We have explained Fast and slow pointer technique in Linked List which is also known as tortoise and hare algorithm. If the linked list has a loop the two pointers will meet, otherwise one of the pointers will eventually become null (when it has reached the end). If there is no cycle, return null. Let us understand:What a circular linked list Jul 11, 2025 · Finding the Start of the Cycle Once the slow and fast pointers meet within the loop, the next step is to identify the start of the cycle. If the fast pointer ever catches up to the slow pointer, there is a loop in the linked list. Conclusion The fast and slow pointers technique is a powerful tool in any programmer’s arsenal, especially when dealing with linked list problems. Its ability to solve complex problems with linear time complexity and constant space complexity makes it an invaluable approach in coding interviews and real-world programming scenarios. Apr 9, 2024 · In Python programming, particularly when manipulating linked lists or arrays, mastering the “ slow and fast pointer ” technique can significantly enhance your problem-solving capabilities. slow = slow -> next and fast pointer by two nodes i. This is useful for finding cycles in linked lists. Yes, if the linked list contains a cycle, the fast and slow pointer will always meet at some point. Dec 11, 2022 · How to use fast and slow pointer technique to solve a pattern of problems of linked list. Assume fast traveled n loops in cycle - 2H + 2D = H + D + L - H + D = nL - H = nl - D g. The two pointers will meet at a node. Write a Linked list class to create a linked list with the help of loop rather than explicitly setting next pointers. 📌 Feb 12, 2024 · Middle Element in Linked List : Fundamental example of fast and slow pointer !! Method 1: Traverse the whole linked list and count the no. The fast and slow pointers technique involves using two pointers that traverse a linked list at different speeds. By mastering this technique, you’ll be well-equipped to tackle many common interview Jun 18, 2020 · 95% of all singly linked list problems are solved with some type of pointers. To do this, we reset one of the pointers - let’s say the slow pointer to the head of the linked list. , fast == NULL or fast->next == NULL), there is no loop in the linked list. If you've ever wondered how to detect cycles in linked lists, find middle elements without counting, or solve complex problems with surprising efficiency, you're about to discover your new favorite technique. Jan 30, 2021 · Example Implementation Here's how to use this algorithm for the Leetcode problem: Linked List Cycle. What Are Slow and Fast Pointers? Slow and fast pointers is a technique where two pointers traverse a data structure at different speeds. Reverse the second half of the list. When slow catches fast, slow traveled H + D and fast traveled 2(H + D) f. MOST ASKED DSA QUESTIONS - Free download as PDF File (. In this video, we learn how to check if a linked list is circular using the two pointer (slow & fast) approach. of nodes. The slow and fast pointers algorithm (also known as Floyd's Cycle Detection algorithm or the Tortoise and Hare algorithm) uses two pointers to determine traits about a graph. Master this technique to solve half of linked list problems easily on platforms like Nov 24, 2023 · Fast and slow pointers are a common pattern used in algorithmic problem-solving, particularly in linked list and two-pointer problems. Jul 23, 2025 · The faster one is called the fast pointer and the other one is called the slow pointer. In this example, we are going to use two pointers to find the mid point of a linked list. Given two pointers, named slow and fast, both start at the head of the list. . If there’s a cycle, the fast pointer will eventually meet the slow pointer; if not, the fast pointer will reach the end of the list. Oct 31, 2022 · In the Fast & Slow Pointers technique, two pointers start at the same position and iterate through an array (or linked list) at different speeds. There is a cycle in a linked list if there is some node in the list that can be reached again by continuously following the next pointer. Leetcode 141 - Intiution behind Floyd's Cycle Detection in Linked List (with Slow and Fast Pointers) - Detect Loop in a Linked ListLeetcode 141: Linked List Dec 12, 2023 · Data Structures and Algorithms. io/ - A better way to prep for Coding InterviewsToday we're taking a look at Leetcode 141 Linked List Cycle. While traversing the linked list one of these things will occur- The Fast pointer may reach the end (NULL) which shows that there is no loop in the linked list. Jan 10, 2026 · 🚀 Ultimate Linked List Master List for Amazon SDE-1 Prep (Deduplicated & Grouped Logically)Hey LeetCoders! 👋If you're grinding for Amazon SDE-1 (or any FAANG-level role), Linked Lists are a staple in interviews—especially for pointer manipulation, cy X_____/ e. This Day 11 of My 90-Day DSA Pattern Mastery Challenge! Today I continued my deep dive into the Fast & Slow Pointer Pattern — and solved a very interesting medium-level problem Circular Array Loop The Detect Cycle in Linked List problem is a classic DSA question that tests pointer manipulation and optimization skills. The slow pointer moves one step at a time while the fast pointer moves two steps. 🚀 Day 32 of My DSA Journey (Python + LeetCode) Today’s focus was on a classic Linked List problem that combines multiple patterns — checking whether a linked list is a palindrome. e. Compare the values of nodes from the first half with the reversed second half. In this video, I solve LeetCode Problem 141: Linked List Cycle using the Fast & Slow Pointer approach (also known as Floyd’s Tortoise and Hare Algorithm). Fast/Slow pointers and Reversing Hi, hope you’re doing well at the moment! Today we’re going to dive into one of the tricky topics well-known as … The algorithm works as follows: Use slow and fast pointers to locate the middle of the linked list. txt) or read online for free. 6K subscribers Subscribe Swap Nodes in Pairs || Recursive & Iterative || Linked List || C++/Java/Python || Leetcode 24 10+ UNIQUE Projects for Job || Projects for Internship and Placement || Project Ideas by Aryan In this tutorial, we're going to do a deep dive on one such algorithm. I'm going to show you Jul 31, 2023 · Else, move the slow pointer by one node i. Day 10 of My 90-Day DSA Pattern Mastery Challenge! Today I solved a single medium-level problem — but one that perfectly tested how well I truly understand the Fast & Slow Pointer Pattern: 1 Optimized Approach (Two Pointers + Reverse) Use slow and fast pointers to find the middle of the list. Dec 9, 2025 · Master the tortoise and hare algorithm with comprehensive templates, mathematical proofs, and real-world applications. Reverse the second half of the linked list starting from the node after the slow pointer. Dec 17, 2021 · The fast pointer moves with double the speed of the slow pointer, that's why when the fast pointer reaches the end of the linked list, the slow pointer is in the middle. Jul 18, 2023 · Read more here Q. pdf), Text File (. This pattern involves maintaining two pointers that traverse a data structure (like an array or linked list) at different speeds. Linked List. Given the head of a linked list, determine if the linked list has a cycle in it. Nov 22, 2025 · This problem is commonly asked to test linked list understanding, pointers, and in-place transformations. Jul 23, 2025 · Move the fast pointer two nodes at a time, while moving the slow pointer one node at a time. Otherwise, the fast and slow pointers will eventually meet at some point, indicating a loop in the linked list. Sep 17, 2025 · 2. 2: Will the fast and slow pointer always meet at some point if the list contains a cycle? Ans. Being able to succinctly perform these tasks in a timely fashion and communicate your ideas in a coherent manner is what can make the difference in thousands of dollars of Connect with me: / pratyushnarain55555 / padho_with_pratyush The slow fast pointer dsa pattern is one of the most powerful patterns while dealing with linked list questions. 🚀 What You'll Learn Intuition behind twin pairs How fast & slow pointers help in leetcode, coding interview question, data structures, data structures and algorithms, linked lists, fast and slow pointers In Lecture 7, we'll dive into the magic of fast and slow pointers in software engineering and data structures. Can you solve this real interview question? Linked List Cycle II - Given the head of a linked list, return the node where the cycle begins. Jan 19, 2022 · When using the method of fast and slow, in a linkedlist which has loop. Learn cycle detection, finding middle elements, and advanced techniques. Middle of the Linked List - Leetcode 876 - Linked Lists (Python) This Algorithm is SUPER HELPFUL for Coding Interviews! | Fast & Slow Pointers for Linked Lists 13K Dislike Jul 22, 2024 · The fast and slow pointer technique is a powerful tool for solving various linked list and array problems. The fast pointer typically moves faster than the slow pointer. The fast pointer should catch the slow pointer once both the pointers are in a cyclic loop. When the fast pointer reaches the end, the slow pointer will be about halfway.

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