计数算法-对200万考生的成绩就行排序

import java.io.*;
import java.text.DecimalFormat;


/**
 * @author: small sunshine
 * @Description：
 * @date: 2021/6/20 10:04 上午
 */
public class ScoreDescSort {

    private static long length = 200 * 10000;

    private static int[] score = new int[200 * 10000];

    private static int[] scoreCopy = new int[750 * 100 + 1];

    private static File file = new File("/Users/lizhenguang/Desktop/score.txt");

    private static File fileCopy = new File("/Users/lizhenguang/Desktop/score_copy.txt");

    /**
     * 对200万考生的分数（保留两位小数）倒序排列
     *
     * @param args
     * @throws IOException
     */
    public static void main(String[] args) throws IOException {
        long start = System.currentTimeMillis();
        createFile(length);
        createArray();
        flushArray();
        printResult();
        long end = System.currentTimeMillis();
        System.out.println("操作总用时：" + (end - start) + "ms");
    }

    /**
     * 1、创建分数文件
     *
     * @param length
     * @throws IOException
     */
    public static void createFile(long length) throws IOException {

        OutputStreamWriter outputStreamWriter = new OutputStreamWriter(new FileOutputStream(file), "UTF-8");
        BufferedWriter bufferedWriter = new BufferedWriter(outputStreamWriter);
        for (int i = 0; i < length; i++) {
            if (i != 0) {
                bufferedWriter.newLine();
            }
            bufferedWriter.write(doubleFormat(Math.random() * 750));
        }
        bufferedWriter.flush();
        outputStreamWriter.flush();
        bufferedWriter.close();
        outputStreamWriter.close();
    }

    /**
     * 从文件中读分数并写入到数组中
     *
     * @throws IOException
     */
    public static void createArray() throws IOException {
        InputStreamReader inputStreamReader = new InputStreamReader(new FileInputStream(file));
        BufferedReader bufferedReader = new BufferedReader(inputStreamReader);
        String scores = null;
        int index = 0;
        while ((scores = bufferedReader.readLine()) != null) {
            score[index++] = (int) (Double.valueOf(scores) * 100);
        }
        bufferedReader.close();
        inputStreamReader.close();
    }

    /**
     * 计数法处理分数
     */
    public static void flushArray() {
        for (int i:  score) {
            scoreCopy[i]++;
        }
    }

    /**
     * 排好序并打印出来
     *
     * @throws IOException
     */
    public static void printResult() throws IOException{
        OutputStreamWriter outputStreamWriter = new OutputStreamWriter(new FileOutputStream(fileCopy), "UTF-8");
        BufferedWriter bufferedWriter = new BufferedWriter(outputStreamWriter);
        for (int j = scoreCopy.length - 1; j >= 0; j--) {
            if (scoreCopy[j] > 0) {
                for (int z = 0; z < scoreCopy[j]; z++) {
                    bufferedWriter.write(doubleFormat((j / 100D)) + "");
                    bufferedWriter.newLine();
                }
            }
        }
        bufferedWriter.flush();
        outputStreamWriter.flush();
        bufferedWriter.close();
        outputStreamWriter.close();

    }



    /**
     * 随机分数-格式化：#.##
     *
     * @param in
     * @return
     */
    public static String doubleFormat(double in) {
        DecimalFormat decimalFormat =  new DecimalFormat();
        decimalFormat.applyPattern("0.00");
        return decimalFormat.format(in);
    }
}