Easy Problems -

1. Lemonade Change
2. Shortest Distance to a Character
3. Greatest Common Divisor of Strings
4. Design HashMap
5. Logger Rate Limiter
6. Backspace String Compare
7. Valid Parentheses
8. Leaf-Similar Trees
9. Maximum Subarray
10. Symmetric Tree
11. Valid Palindrome II
12. Diameter of Binary Tree
13. Best Time to Buy and Sell Stock
14. Single Number
15. Merge Two Sorted Lists

Medium Problems -

1. Time Based Key-Value Store
2. Design Hit Counter
3. Maximum Average Subtree
4. Letter Combinations of a Phone Number
5. Dice Roll Simulation
6. Rank Teams by Votes
7. Longest Increasing Subsequence
8. Number of Islands
9. Single Element in a Sorted Array
10. Peeking Iterator
11. Evaluate Reverse Polish Notation
12. Meeting Rooms II
13. Decode String

14. Friend Circles
15. Validate Binary Search Tree
16. Subarray Product Less Than K
17. Next Permutation
18. Palindromic Substrings
19. Flatten Nested List Iterator
20. Permutations
21. Insert into a Binary Search Tree
22. Random Pick with Weight
23. Basic Calculator II
24. Longest Substring Without Repeating Characters
25. Binary Search Tree Iterator

Hard Problems -

1. Bus Routes
2. Frog Jump
3. Sliding Window Maximum
4. Minimum Difficulty of a Job Schedule
5. Shortest Distance from All Buildings
6. Optimal Account Balancing
7. Find Median from Data Stream
8. Best Time to Buy and Sell Stock III
9. Merge k Sorted Lists
10. Find the Closest Palindrome
11. Minimum Window Substring
12. Maximum Frequency Stack
13. Department Top Three Salaries
14. Basic Calculator
15. Median of Two Sorted Arrays