Linked List Problems

1. Linked List Cycle
   Problem: Determine if a linked list has a cycle.
   Difficulty: Easy

2. Linked List Cycle II
   Problem: Find the node where the cycle begins in a linked list.
   Difficulty: Medium

3. Happy Number
   Problem: Determine if a number is a "happy number," which involves detecting cycles in the sum of the squares of its digits.
   Difficulty: Easy

4. Middle of the Linked List
   Problem: Find the middle node of a linked list.
   Difficulty: Easy

5. Palindrome Linked List
   Problem: Determine if a linked list is a palindrome.
   Difficulty: Easy

6. Intersection of Two Linked Lists
   Problem: Find the intersection node of two singly linked lists.
   Difficulty: Easy
   Array Problems

7. Find the Duplicate Number
   Problem: Find the duplicate number in an array where the numbers are used to form a cycle.
   Difficulty: Medium

8. Circular Array Loop
   Problem: Determine if there is a loop in an array where each index points to the next index to visit.
   Difficulty: Medium

9. Circular Array Loop
   Problem: Similar to problem 142, but on arrays instead of linked lists.
   Difficulty: Medium

10. Remove Duplicates from Sorted Array
    Problem: Use two pointers to remove duplicates from a sorted array in place.
    Difficulty: Easy

11. Sort Colors
    Problem: Sort an array of colors using a two-pointer technique.
    Difficulty: Medium

12. Remove Nth Node From End of List
    Problem: Remove the nth node from the end of a linked list using two pointers.
    Difficulty: Medium

13. Longest Substring Without Repeating Characters
    Problem: Find the length of the longest substring without repeating characters.
    Difficulty: Medium
    Two Pointers (Related Technique)

14. Two Sum II - Input Array Is Sorted
    Problem: Find two numbers in a sorted array that add up to a specific target using a two-pointer technique.
    Difficulty: Medium

15. 3Sum
    Problem: Find all unique triplets in an array that sum up to zero using a two-pointer technique.
    Difficulty: Medium

16. 3Sum Closest
    Problem: Find a triplet that is closest to a given target using a two-pointer technique.
    Difficulty: Medium

17. Merge Sorted Array
    Problem: Merge two sorted arrays into one using two pointers.
    Difficulty: Easy

18. Longest Word in Dictionary through Deleting
    Problem: Find the longest word in a dictionary that can be formed by deleting some characters of a given string using two pointers.
    Difficulty: Medium