#+ANKI_DECK: study_deck_02
#+TITLE: DSA Tricks & Patterns

* Task: Count character frequencies faster than map :dsa:counting:array:string:retrieval::production:
:PROPERTIES:
:ANKI_NOTE_TYPE: Basic
:ROADMAP: [[file:../roadmap.org::*0242. Valid Anagram][0242. Valid Anagram]]
:END:

** Front
Write C++ to count character frequencies in a string using a
fixed-size array instead of ~std::map~ (assume lowercase a-z only).

** Back
#+begin_src cpp
std::array<int, 26> freq{};
for (char c : s) freq[c - 'a']++;
#+end_src

*Why:* O(1) per access vs O(log n) for ~std::map~. Use when alphabet
is bounded and small.