Rust: What is the difference between HashMap and BTreeMap?

1use std::collections::{HashMap, BTreeMap};
2
3let mut hash = HashMap::new();
4hash.insert(3, "c");
5hash.insert(1, "a");
6
7let mut btree = BTreeMap::new();
8btree.insert(3, "c");
9btree.insert(1, "a");
10// BTreeMap iterates in order: (1, "a"), (3, "c")