What Is the Difference Between HashMap and BTreeMap in Rust?

HashMap provides faster O(1) access with unordered keys, while BTreeMap offers O(log n) access with keys automatically sorted by their Ord implementation.

HashMap offers O(1) average access time with unordered keys, while BTreeMap provides O(log n) access with keys sorted by their Ord implementation. Use HashMap for speed when order doesn't matter, and BTreeMap when you need sorted iteration or range queries.

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

// HashMap: Fast, unordered
let mut hash_map = HashMap::new();
hash_map.insert("apple", 10);
hash_map.insert("banana", 20);

// BTreeMap: Slower, sorted keys
let mut tree_map = BTreeMap::new();
tree_map.insert("apple", 10);
tree_map.insert("banana", 20);
// Iterating tree_map yields keys in alphabetical order

HashMap is like a phone book where you can find a number instantly but the names aren't in order. BTreeMap is like a dictionary where names are always sorted alphabetically, making it slightly slower to find a specific entry but perfect for reading lists in order. Choose HashMap for speed and BTreeMap for sorted data.