Counting

Easy | Array, Hash Table, Counting

Medium | Array, Hash Table, Greedy, Counting

Medium | Hash Table, String, Counting

Hard | Array, Hash Table, Two Pointers, Binary Search, Graph Theory, Sorting, Counting

Easy | Hash Table, String, Counting

Medium | Array, Hash Table, Math, Counting

Hard | Hash Table, Math, String, Combinatorics, Counting

Easy | Array, Counting, Prefix Sum

Hard | Hash Table, String, Dynamic Programming, Graph Theory, Topological Sort, Memoization, Counting

Hard | Array, Math, Binary Search, Counting, Enumeration, Prefix Sum