gistfile1.txt

/// Find first duplicate integer in vector.
/// 
/// Assumed that array a contains only number in range [1, a.len()].
/// O(n) time complexity and O(1) memory. NOT PURE FUNCTIONAL.
///
/// # Example
///
/// ```
/// assert!(first_duplicate(vec![8, 4, 6, 2, 6, 4, 7, 9, 5, 8]) == 6);
/// assert!(first_duplicate(vec![2, 1, 4, 5, 3]) == -1);
/// ```
fn first_duplicate(mut a: Vec<i32>) -> i32 {
    for index in 0..a.len() {
        let number = a[index].abs() as usize;
        let relative_index = number - 1;

        if let Some(elem) = a.get_mut(relative_index) {
            *elem *= -1;

            if elem.is_positive() {
                return number as i32;
            }
        }
    }

    -1
}