What is the difference between BTreeMap and HashMap?

Question

Accepted Answer

**BTreeMap** — sorted keys. **HashMap** — unordered.

**BTreeMap:**
- Sorted by key.
- O(log n) operations.
- Range queries.

**HashMap:**
- Unordered.
- O(1) average.
- Faster lookup.

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

// BTreeMap: sorted
let mut tree = BTreeMap::new();
tree.insert(3, "c");
tree.insert(1, "a");
tree.insert(2, "b");
// Iteration: 1, 2, 3 (sorted)

// HashMap: unordered
let mut hash = HashMap::new();
hash.insert(3, "c");
hash.insert(1, "a");
hash.insert(2, "b");
// Iteration: random order
```

**Key:** BTreeMap for sorted, HashMap for fast lookup.

Rust: What is the difference between BTreeMap and HashMap?