Hashing & Frequency Maps

Three skeletons cover nearly every hashing problem: count, seen-before, and prefix-sum + map.

Two strings are anagrams if every character appears the same number of times in each.

A duplicate exists if we ever see a value we have already stored in a set (or if any frequency exceeds one).

You can build the note only if every character’s demand in ransomNote is covered by the supply in magazine, with each magazine letter used at most once.

Store prefix sum frequencies to find subarrays with target sum in O(n)