Lookups, frequency maps, and the bread-and-butter of interviews.
Return indices of the two numbers that add up to a target.
Cluster words that are rearrangements of the same letters.
Return the k values that appear most often — in O(n) with bucket sort.
Each answer[i] is the product of all other elements — no division, in O(n).
Find the longest run of consecutive integers — in O(n), without sorting.