find the kth largest key in a map in rust

To find the kth largest key in a map in Rust, we need to sort the keys in descending order and then select the kth largest key. We can achieve this using the sort_by_key method of the map, along with a binary search to find the kth largest key. Here's the code:


main.rs
use std::collections::BTreeMap;

fn find_kth_largest_key(map: &BTreeMap<i32, i32>, k: usize) -> Option<&i32> {
    let mut keys: Vec<&i32> = map.keys().collect();
    keys.sort_by_key(|&a| -a); // Sort keys in descending order
    keys.get(k - 1).copied() // Get kth largest key, if it exists
}

fn main() {
    let mut map = BTreeMap::new();
    map.insert(1, 10);
    map.insert(2, 20);
    map.insert(3, 30);

    let k = 2;
    let kth_largest_key = find_kth_largest_key(&map, k);
    match kth_largest_key {
        Some(key) => println!("The {}th largest key is {}", k, key),
        None => println!("Could not find the {}th largest key", k),
    }
}
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In the find_kth_largest_key function, we first collect the keys of the map into a vector, sort the vector by key values in descending order using the sort_by_key method. Finally, we return the kth largest key from the sorted vector using get and copied methods.

Note that we use a BTreeMap here instead of a HashMap as the former is always sorted.

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