用mapreduce实现二分法的代码

二分法是一种常见的查找算法，可以用 MapReduce 实现。下面是一个用 MapReduce 实现二分法的示例代码：

Mapper 阶段：

public class BinarySearchMapper extends Mapper<LongWritable, Text, Text, Text> {
    private int target;

    @Override
    protected void setup(Context context) throws IOException, InterruptedException {
        // 从配置中获取目标数
        target = context.getConfiguration().getInt("target", -1);
    }

    @Override
    protected void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException {
        // 将输入的数据转换成整数
        int num = Integer.parseInt(value.toString());

        // 二分查找
        int left = 0, right = num - 1;
        while (left <= right) {
            int mid = (left + right) / 2;
            if (mid == target) {
                context.write(new Text("result"), new Text(String.valueOf(mid)));
                break;
            } else if (mid < target) {
                left = mid + 1;
            } else {
                right = mid - 1;
            }
        }
    }
}

Reducer 阶段：

public class BinarySearchReducer extends Reducer<Text, Text, Text, Text> {
    @Override
    protected void reduce(Text key, Iterable<Text> values, Context context) throws IOException, InterruptedException {
        for (Text value : values) {
            context.write(key, value);
        }
    }
}

Driver 阶段：

public class BinarySearchDriver {
    public static void main(String[] args) throws Exception {
        Configuration conf = new Configuration();
        conf.setInt("target", 50); // 设置目标数

        Job job = Job.getInstance(conf, "Binary Search");
        job.setJarByClass(BinarySearchDriver.class);
        job.setMapperClass(BinarySearchMapper.class);
        job.setReducerClass(BinarySearchReducer.class);
        job.setOutputKeyClass(Text.class);
        job.setOutputValueClass(Text.class);

        FileInputFormat.addInputPath(job, new Path(args[0]));
        FileOutputFormat.setOutputPath(job, new Path(args[1]));

        System.exit(job.waitForCompletion(true) ? 0 : 1);
    }
}

以上代码仅供参考，实际使用时还需要根据具体情况进行修改。