问题：从data.bat文件中排序，找到数量最大的10个数字，以println的方式分别输出。

Posted 2021-02-28 ludengfu

问题：从data.bat文件中排序，找到数量最大的10个数字，以println的方式分别输出。

要求：
0.时间尽量短
1.类名为Test.java
2.无包名
3.无第三方引用
4.可直接javac编译和java运行
5.java命令行运行class，参数为data.bat文件路径，例如，java Test ./data.bat
6.jdk1.8
7.文件字符集，utf-8
8.不能有异常和警告

data.bat  地址：https://github.com/TabMM/drafts/tree/master/data.bat

废话不多说 直接上代码 （未经允许，不得转载）

方法一（本人写）：

public class Test {
    public static void main(String[] args) {
        long start = System.currentTimeMillis();
        readFileByLines();
        System.err.print("使用时间" + (System.currentTimeMillis() - start) + "ms");
    }

    /**
     * 以行为单位读取文件，常用于读面向行的格式化文件
     */

    public static void readFileByLines() {
        int[] s = new int[]{0, 0, 0, 0, 0, 0, 0, 0, 0, 0};
        File file = new File("F:\IdeaProjects\snowflake\src\main\java\com\example\snowflake\data.dat");
        BufferedReader reader = null;
        try {
            reader = new BufferedReader(new FileReader(file));
            String tempString = null;
            int line = 1;
            // 一次读入一行，直到读入null为文件结束
            while ((tempString = reader.readLine()) != null) {
                int temp = Integer.parseInt(tempString);
                int t = temp > s[0] ? jh(0, temp, s) : (temp > s[1] ? jh(1, temp, s) : (temp > s[2] ? jh(2, temp, s) : (temp > s[3] ? jh(3, temp, s) : (temp > s[4] ? jh(4, temp, s) : (temp > s[5] ? jh(5, temp, s) : (temp > s[6] ? jh(6, temp, s) : (temp > s[7] ? jh(7, temp, s) : (temp > s[8] ? jh(8, temp, s) : (temp > s[9] ? jh(9, temp, s) : 0)))))))));
                line++;
            }
            System.out.println(s[0] + "
" + s[1] + "
" + s[2] + "
" + s[3] + "
" + s[4] + "
" + s[5] + "
" + s[6] + "
" + s[7] + "
" + s[8] + "
" + s[9]);
            reader.close();
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            if (reader != null) {
                try {
                    reader.close();
                } catch (IOException e1) {
                }
            }
        }
    }

    private static int jh(int index, int temp, int[] s) {
        int a = index;
        for (int i = 0; i < 9 - index; i++) {
            stamp(i, s);
        }
        s[a] = temp;
        return 0;
    }

    private static void stamp(int index, int[] s) {
        s[9 - index] = s[8 - index];
    }

}

方法二（@小和写）：

public class Demo {
    public static void main(String[] args) throws Exception {
        long start = System.currentTimeMillis();
        TreeMap<Integer,Integer> treeMap = new TreeMap<>(new Comparator<Integer>() {
            @Override
            public int compare(Integer o1, Integer o2) {
                return o2.compareTo(o1);
            }
        });
        BufferedReader br=new BufferedReader(new FileReader("F:\IdeaProjects\snowflake\src\main\java\com\example\snowflake\data.dat"));//读取文件
        //输出
        for(String s=br.readLine();s!=null;s=br.readLine()) {
            treeMap.put(Integer.parseInt(s), Integer.parseInt(s));
            if(treeMap.size() > 10){
                treeMap.remove(treeMap.lastKey());
            }
        }
        Iterator<Map.Entry<Integer,Integer>> it = treeMap.entrySet().iterator();
        while(it.hasNext()) {
            Map.Entry<Integer,Integer> entry = it.next();
            System.out.println(entry.getKey() + " : " + entry.getValue());
        }
        System.err.print( "使用时间"+(System.currentTimeMillis() - start) +"ms");
    }
}
方法三（@咿呀咿呀哟写）：

public class Test {

    public static int N = 10;           // Top10
    public static int arr[] = new int[N];
    //数组长度
    public static int len = arr.length;
    //堆中元素的有效元素 heapSize<=len
    public static int heapSize = len;

    public static void main(String[] args){
        long start = System.currentTimeMillis();
        int _150M = 1024*1024*150;
        File fin = new File("F:\IdeaProjects\snowflake\src\main\java\com\example\snowflake\data.dat");
        FileChannel fcin = null;
        try {
            fcin = new RandomAccessFile(fin, "r").getChannel();
        } catch (FileNotFoundException e) {
            System.out.println("系统找不到指定文件");
            return;
        }
        ByteBuffer rBuffer = ByteBuffer.allocate(_150M);
        readFileByLine(_150M, fcin, rBuffer);
        System.err.print( "使用时间"+(System.currentTimeMillis() - start) +"ms");
    }
    public static void readFileByLine(int bufSize, FileChannel fcin,
                                      ByteBuffer rBuffer) {
        String enterStr = "
";
        try {
            byte[] bs = new byte[bufSize];
            String tempString = null;
            while (fcin.read(rBuffer) != -1) {
                int rSize = rBuffer.position();
                rBuffer.rewind();
                rBuffer.get(bs);
                rBuffer.clear();
                tempString = new String(bs, 0, rSize);
                int fromIndex = 0;
                int endIndex = 0;
                int  i = 0 ;
                while ((endIndex = tempString.indexOf(enterStr, fromIndex)) != -1) {
                    String line = tempString.substring(fromIndex, endIndex);
                    fromIndex = endIndex + 1;
                    if (i<10){
                        arr[i]= Integer.valueOf(line);
                        i++;
                        continue;
                    }
                    //构建小顶堆
                    buildMinHeap();
                    if(Integer.valueOf(line) > arr[0]){
                        arr[0] = Integer.valueOf(line);
                        minHeap(0);
                    }
                }
                print();
            }
        } catch (IOException e) {
            System.out.println("读取文件失败");
            return;
        }
    }
    /**
     * 自底向上构建小堆
     */
    public static void buildMinHeap(){
        int size = len / 2;
        for(int i = size;i>=0;i--){
            minHeap(i);
        }
    }

    /**
     * i节点为根及子树是一个小堆
     * @param i
     */
    public static void minHeap(int i){
        int l = left(i);
        int r = right(i);
        int index = i;
        if(l<heapSize && arr[l]<arr[index]){
            index = l;
        }
        if(r<heapSize && arr[r]<arr[index]){
            index = r;
        }
        if(index != i){
            int t = arr[index];
            arr[index] = arr[i];
            arr[i] = t;
            //递归向下构建堆
            minHeap(index);
        }
    }

    /**
     * 返回i节点的左孩子
     * @param i
     * @return
     */
    public static int left(int i){
        return 2*i;
    }

    /**
     * 返回i节点的右孩子
     * @param i
     * @return
     */
    public static int right(int i){
        return 2*i+1;
    }
    /**
     * 打印
     */
    public  static void print(){
        for (int i = arr.length -1; i >=0 ; i--) {
            System.out.println(arr[i]);
        }
    }
}