計數排序

import java.util.Arrays;
import java.util.Random;

public class CountSort {

    static class Item {
        int index;
        int num;
    }

    private static void sort(Item[] items, int bound) {

        int n = items.length;

        int[] count = new int[bound];
        int[] sum = new int[bound];

        Item[] cache = Arrays.copyOf(items, n);

        for (int i = 0; i < n; ++ i) {
            count[items[i].num] ++;
        }

        sum[0] = count[0];

        for (int i = 1; i < bound; ++ i) {
            sum[i] = sum[i - 1] + count[i];
        }

        for (int i = n - 1; i >= 0; -- i) {
            items[-- sum[cache[i].num]] = cache[i];
        }
    }

    public static void main(String[] args) {

        int n = 10;

        Random random = new Random();

        Item[] items = new Item[n];

        for (int i = 0; i < n; ++ i) {
            items[i] = new Item();
            items[i].index = i;
            items[i].num = random.nextInt(5);
        }

        Arrays.stream(items).forEach(item -> {
            System.out.println(item.index + "->" + item.num);
        });
        sort(items, 5);
        System.out.println("排序後");
        Arrays.stream(items).forEach(item -> {
            System.out.println(item.index + "->" + item.num);
        });
    }
}