HashMap/HashSet

Easy

1. Two Sum
136. Single Number
169. Majority Element
205. Isomorphic Strings
217. Contains Duplicate
219. Contains Duplicate II
242. Valid Anagram
290. Word Pattern
299. Bulls and Cows
349. Intersection of Two Arrays
350. Intersection of Two Arrays II
383. Ransom Note
387. First Unique Character in a String
389. Find the Difference
409. Longest Palindrome
451. Sort Characters By Frequency
500. Keyboard Row
705. Design HashSet
706. Design HashMap
748. Shortest Completing Word
771. Jewels and Stones
804. Unique Morse Code Words
819. Most Common Word
961. N-Repeated Element in Size 2N Array
1002. Find Common Characters
1122. Relative Sort Array
1189. Maximum Number of Balloons

Medium

49. Group Anagrams
128. Longest Consecutive Sequence
146. LRU Cache
159. Longest Substring with At Most Two Distinct Characters
187. Repeated DNA Sequences
208. Implement Trie (Prefix Tree)
211. Design Add and Search Words Data Structure
274. H-Index
347. Top K Frequent Elements
388. Longest Absolute File Path
438. Find All Anagrams in a String
454. 4Sum II
560. Subarray Sum Equals K
567. Permutation in String
763. Partition Labels
792. Number of Matching Subsequences
1081. Smallest Subsequence of Distinct Characters

Hard

30. Substring with Concatenation of All Words
76. Minimum Window Substring
316. Remove Duplicate Letters