use std::collections::HashMap;
use std::hash::Hash;
/// Finds the element with the minimum number of occurrences in a slice.
///
/// # Generics
/// * `T`: Must implement `Eq` and `Hash` to be stored in a `HashMap`,
/// and `Clone` to return a copy of the value.
///
/// # Returns
/// * `Option<T>`: `Some(element)` containing the least frequent item, or `None` if the slice is empty.
pub fn min_occurrence<T>(slice: &[T]) -> Option<T>
where
T: Eq + Hash + Clone,
{
if slice.is_empty() {
return None;
}
// 1. Tally element frequencies using HashMap and Entry API in O(N) time
let mut counts = HashMap::with_capacity(slice.len());
for item in slice {
*counts.entry(item).or_insert(0usize) += 1;
}
// 2. Find the key with the minimum frequency count using iterator combinators
counts
.into_iter()
.min_by_key(|&(_, count)| count)
.map(|(item, _)| item.clone())
}
fn main() {
let vec = vec![1, 3, 1, 1, 4, 4, 5, 5, 4, 2, 2, 2, 3, 3, 3];
match min_occurrence(&vec) {
Some(result) => println!("{result}"), // Output: 5
None => println!("The slice was empty."),
}
}
/*
run:
5
*/
