rs-01-rle

# Run-length encoding

Implement the library file **`src/lib.rs`** exposing:

```rust
pub fn run_length_encode(input: &str) -> String
```

Encode consecutive runs of the same character as `<char><count>`. A single character still
gets a count of `1`.

- `run_length_encode("aaabbc")` → `"a3b2c1"`
- `run_length_encode("")` → `""`
- `run_length_encode("abc")` → `"a1b1c1"`

Tests live in `tests/` and call it as `challenge::run_length_encode`.

use challenge::run_length_encode;

#[test]
fn basic_runs() {
    assert_eq!(run_length_encode("aaabbc"), "a3b2c1");
}

#[test]
fn empty() {
    assert_eq!(run_length_encode(""), "");
}

#[test]
fn all_singles() {
    assert_eq!(run_length_encode("abc"), "a1b1c1");
}

```rust
pub fn run_length_encode(input: &str) -> String {
    let mut result = String::new();
    let mut chars = input.chars();
    
    if let Some(mut current) = chars.next() {
        let mut count = 1;
        for c in chars {
            if c == current {
                count += 1;
            } else {
                result.push(current);
                result.push_str(&count.to_string());
                current = c;
                count = 1;
            }
        }
        result.push(current);
        result.push_str(&count.to_string());
    }
    
    result
}
```