0347-top-k-frequent-elements.rs

use std::collections::HashMap;
use std::cmp::Ordering;

impl Solution {
    pub fn top_k_frequent(nums: Vec<i32>, k: i32) -> Vec<i32> {
        let mut map: HashMap<i32, i32> = HashMap::new();
        
        for n in nums{
            *map.entry(n).and_modify(|val| *val+=1).or_default();
        }
        
        let mut freq: Vec<(i32, i32)> = map.into_iter().collect();

        let res = match (k as usize).cmp(&freq.len()){
            Ordering::Equal => &freq,
            _ => quick_select(&mut freq, k),
        };
        
        res.into_iter()
        .map(|&n| n.0)
        .collect()
    }
}

pub fn quick_select(slice: &mut [(i32, i32)], k: i32) -> &[(i32, i32)]{
    let (mut pivot, mut i, mut j) = (0, 1, 1);
    
    for index in 1..slice.len(){
        if slice[index].1 >= slice[pivot].1{
            slice.swap(index, j);
        }else{
            slice.swap(index, i);
            i+=1;
        }
        j+=1;
    }
    
    slice.swap(pivot, i - 1);
    pivot = i - 1;
    
    let larger_items = (j - pivot) as i32;
    
    match larger_items.cmp(&k) {
        Ordering::Less => quick_select(&mut slice[0..j], k),
        Ordering::Greater => quick_select(&mut slice[pivot + 1..j], k),
        Ordering::Equal => &slice[pivot..j],
    }
}