How to use BTreeMap vs HashMap

Use HashMap for fastest average-case lookups when key order doesn't matter, and BTreeMap when you need keys sorted or deterministic iteration order.

use std::collections::{HashMap, BTreeMap};

// Fastest average lookup, random order
let mut unordered = HashMap::new();
unordered.insert("apple", 1);

// Sorted keys, deterministic iteration
let mut ordered = BTreeMap::new();
ordered.insert("apple", 1);

HashMap uses a hash function for O(1) average access but offers no ordering guarantees. BTreeMap keeps keys sorted in a tree structure, providing O(log n) access and iteration in key order.

Choose HashMap if you need the fastest possible lookups and don't care about the order of your data. Choose BTreeMap if you need your data to stay sorted automatically or if you need the same order every time you run your program. Think of HashMap as a fast but messy desk drawer, and BTreeMap as a neatly organized filing cabinet.