# ๐Ÿ”— Assignment 11 โ€” Linked Lists > **Lecture:** Topic 11 โ€” Linked Lists Foundations > **Topic Count:** 11 of 30 โ€” Phase 2 Core Data Structures > **Duration:** 6 Days > **Core Problems:** 28 (10 Easy ยท 13 Medium ยท 5 Hard) > **Goal:** Master pointer rerouting, Fast & Slow pointer paradigms, and complex cache design invariants (LRU/LFU). --- ## ๐Ÿ“Š Topic Overview Linked Lists are the foundational bridge between linear data structures (Arrays) and hierarchical ones (Trees). Mastering them requires a mental shift from "index-based access" to **"reference-based navigation"**. - **Primary Patterns**: Fast & Slow Pointers, In-place Reversal, Sentinel Head (Dummy), Interweaving, Frequency Mapping. - **Key Focus**: Maintaining pointer integrity during complex re-linking. --- ## ๐ŸŸข Easy Tier (Foundation & Floyd's) _Focus on traversing, basic manipulation, and the "Tortoise and Hare" strategy._ 1. **[Reverse Linked List](https://leetcode.com/problems/reverse-linked-list/) (LC 206)** โ€” The fundamental iterative vs recursive question. `[Pattern: Reversal]` 2. **[Middle of the Linked List](https://leetcode.com/problems/middle-of-the-linked-list/) (LC 876)** โ€” Standard Fast & Slow pointer application. `[Pattern: Fast & Slow]` 3. **[Linked List Cycle](https://leetcode.com/problems/linked-list-cycle/) (LC 141)** โ€” Detecting cycles using meeting points. `[Pattern: Floyd's Detection]` 4. **[Merge Two Sorted Lists](https://leetcode.com/problems/merge-two-sorted-lists/) (LC 21)** โ€” Use a Dummy Head to simplify edge cases. `[Pattern: Dummy Head]` 5. **[Delete Node in a Linked List](https://leetcode.com/problems/delete-node-in-a-linked-list/) (LC 237)** โ€” The "O(1) Copy Value" interview trick. `[Pattern: Value Override]` 6. **[Remove Linked List Elements](https://leetcode.com/problems/remove-linked-list-elements/) (LC 203)** โ€” Clean deletion with Dummy Head. `[LC Easy]` 7. **[Palindrome Linked List](https://leetcode.com/problems/palindrome-linked-list/) (LC 234)** โ€” Combine Middle + Reverse + Compare. `[Pattern: Composition]` 8. **[Intersection of Two Linked Lists](https://leetcode.com/problems/intersection-of-two-linked-lists/) (LC 160)** โ€” Two pointers or length difference method. `[Company: Amazon]` 9. **[Remove Duplicates from Sorted List](https://leetcode.com/problems/remove-duplicates-from-sorted-list/) (LC 83)** โ€” Standard traversal and pointer skipping. `[LC Easy]` 10. **[Convert Binary Number in a Linked List to Integer](https://leetcode.com/problems/convert-binary-number-in-a-linked-list-to-integer/) (LC 1290)** โ€” Horner's method for binary. `[LC Easy]` --- ## ๐ŸŸก Medium Tier (Advanced Rerouting) _Focus on multi-step logic and complex pointer state management._ 1. **[Remove Nth Node From End of List](https://leetcode.com/problems/remove-nth-node-from-end-of-list/) (LC 19)** โ€” Use two pointers with a gap of N. `[Pattern: Gap Pointers]` 2. **[Linked List Cycle II](https://leetcode.com/problems/linked-list-cycle-ii/) (LC 142)** โ€” Find the start of the cycle using the $2x$ vs $1x$ math. `[Pattern: Floyd's]` 3. **[Reorder List](https://leetcode.com/problems/reorder-list/) (LC 143)** โ€” Middle + Reverse + Alternating Merge. `[Company: Microsoft]` 4. **[Odd Even Linked List](https://leetcode.com/problems/odd-even-linked-list/) (LC 328)** โ€” Group nodes by index parity in $O(N)$ time. `[Pattern: Multi-Pointer]` 5. **[Add Two Numbers](https://leetcode.com/problems/add-two-numbers/) (LC 2)** โ€” The classic "Full Adder" logic on lists. `[Company: Google]` 6. **[Add Two Numbers II](https://leetcode.com/problems/add-two-numbers-ii/) (LC 445)** โ€” Handling reverse arithmetic using stacks. `[LC Medium]` 7. **[Copy List with Random Pointer](https://leetcode.com/problems/copy-list-with-random-pointer/) (LC 138)** โ€” Iterative $O(1)$ space "interweaving" strategy. `[Company: Meta]` 8. **[Sort List](https://leetcode.com/problems/sort-list/) (LC 148)** โ€” Implementing Merge Sort with $O(N \log N)$ stability. `[Pattern: Divide & Conquer]` 9. **[Rotate List](https://leetcode.com/problems/rotate-list/) (LC 61)** โ€” Shifting nodes based on $(k \pmod L)$ index. `[LC Medium]` 10. **[Swapping Nodes in a Linked List](https://leetcode.com/problems/swapping-nodes-in-a-linked-list/) (LC 1721)** โ€” Efficient $O(N)$ node swapping. `[LC Medium]` 11. **[Split Linked List in Parts](https://leetcode.com/problems/split-linked-list-in-parts/) (LC 725)** โ€” Sub-list management and sizing. `[LC Medium]` 12. **[Flatten a Multilevel Doubly Linked List](https://leetcode.com/problems/flatten-a-multilevel-doubly-linked-list/) (LC 430)** โ€” Pointer manipulation on steroids. `[LC Medium]` 13. **[Partition List](https://leetcode.com/problems/partition-list/) (LC 86)** โ€” Using two separate dummy heads for sorting. `[Pattern: Two-Queue]` --- ## ๐Ÿ”ด Challenge Zone (Mastery & Design) _Focus on cache design invariants and k-sized re-grouping._ 1. **[Reverse Nodes in k-Group](https://leetcode.com/problems/reverse-nodes-in-k-group/) (LC 25)** โ€” The ultimate test of pointer control and recursion. `[Pattern: Reversal]` 2. **[Merge k Sorted Lists](https://leetcode.com/problems/merge-k-sorted-lists/) (LC 23)** โ€” Use a Min-Heap/PriorityQueue for $O(N \log K)$. `[Pattern: Heap]` 3. **[Reverse Linked List II](https://leetcode.com/problems/reverse-linked-list-ii/) (LC 92)** โ€” Reversing a bounded sub-segment in one pass. `[LC Medium/Hard]` 4. **[LRU Cache](https://leetcode.com/problems/lru-cache/) (LC 146)** โ€” HashMap + Doubly Linked List for $O(1)$ access. `[Pattern: DLL]` 5. **[LFU Cache](https://leetcode.com/problems/lfu-cache/) (LC 460)** โ€” Least Frequently Used using multiple lists. `[Pattern: Freq Maps]` --- ## ๐Ÿ“Š Conceptual Check 1. **Space Trade-off**: Why is iterative reversal preferred over recursive in production? 2. **Infinite Loops**: What safety check prevents a cycle in a merged list? 3. **Dummy Head**: When is using a sentinel node actually detrimental? 4. **Complexity Check**: Why is random access $O(N)$ for LL but $O(1)$ for Arrays? --- **Next Topic**: Topic 12 โ€” Stacks & Queues โ†’