Counting

Medium | Hash Table, String, Sorting, Counting

Easy | Array, Hash Table, String, Bit Manipulation, Counting

Hard | Array, Hash Table, Dynamic Programming, Bit Manipulation, Counting

Hard | Array, Math, Counting, Number Theory

Hard | Array, Hash Table, Math, Dynamic Programming, Bit Manipulation, Counting, Number Theory, Bitmask

Easy | Array, Hash Table, Counting

Hard | Hash Table, Two Pointers, String, Backtracking, Counting, Enumeration

Medium | Array, Hash Table, String, Counting

Hard | Array, Hash Table, Counting, Enumeration, Prefix Sum

Medium | Array, Hash Table, String, Counting

Medium | Array, Depth-First Search, Breadth-First Search, Union-Find, Matrix, Counting

Easy | Array, Bit Manipulation, Sorting, Counting

Medium | Array, Hash Table, String, Trie, Sorting, Heap (Priority Queue), Bucket Sort, Counting

Medium | Array, Hash Table, String, Counting

Medium | Array, Hash Table, Two Pointers, Sorting, Counting

Medium | Hash Table, String, Counting

Medium | String, Greedy, Counting

Easy | Array, Hash Table, Counting

Medium | Array, Hash Table, Sorting, Counting, Enumeration

Hard | Array, Hash Table, Sorting, Counting, Enumeration