迅雷的一个面试题

import java.io.BufferedOutputStream;
import java.io.DataOutputStream;
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.util.Random;

//@author piaoboyi@gmail.com
public class BinarySearch {
    public static int nCount = (int) 2e8;

    //!!!!!!!!!!!!!!!!!! Choose disk that is large enough !!!!!!!!!!!!!!
    public static String path = new String("D:/data.dat");

    public static String testPath = new String("D:/test.dat");

    public static Random r = new Random();

    public static void BuildFile() throws IOException {
        // Be sure to use buffer.
        DataOutputStream out = new DataOutputStream(new BufferedOutputStream(
                new FileOutputStream(new File(path))));
        int nCurrentScore = 0;
        for (int i = 0; i < nCount; i++) {
            // int nID = r.nextInt(100);
            // nID is not so useful.
            int nID = 12341231;
            out.writeInt(nID);
            // Becareful to generate the score. because score is small that sizeof(int)/2.
            // Which makes random number should < 20.
            nCurrentScore += r.nextInt(5);
            out.writeInt(nCurrentScore);
        }
        out.close();
    }

    public static void BuildTestFile() throws IOException {
        DataOutputStream out = new DataOutputStream(new BufferedOutputStream(
                new FileOutputStream(new File(testPath))));
        // 25|1
        // 31|2
        // 17|3
        // ...
        out.writeInt(25);
        out.writeInt(1);
        out.writeInt(31);
        out.writeInt(2);
        out.writeInt(17);
        out.writeInt(3);

        out.close();
    }

    public static long BSearch(String path, int nScore) throws IOException {
        RandomAccessFile raf = new RandomAccessFile(path, "r");
        assert (raf.length() % 8 == 0);

        long l = 0, r = raf.length() / 8 - 1;
        while (l <= r) {
            long m = (l + r) / 2;
            long pointer = m * 8;
            raf.seek(pointer);

            @SuppressWarnings("unused")
            int id = raf.readInt();// id.
            int score = raf.readInt();

            if (nScore == score) {
                raf.close();
                return m;
            }
            if (nScore < score)
                r = m - 1;
            else
                l = m + 1;
        }

        raf.close();
        return -1L;
    }

    public static void main(String[] args) throws IOException {
        // BuildTestFile();
        long a = System.currentTimeMillis();
        BuildFile();
        long b = System.currentTimeMillis();
        System.out.println((b-a)/(1000));
        System.out.println("Rank is " + BSearch(path, 0));
    }
}