A:B,C,D,F,E,O
B:A,C,E,K
C:F,A,D,I
D:A,E,F,L
E:B,C,D,M,L
F:A,B,C,D,E,O,M
G:A,C,D,E,F
H:A,C,D,E,O
I:A,O
J:B,O
K:A,C,D
L:D,E,F
M:E,F,G
O:A,H,I,J,K

A:B,C,D,F,E,O 

A:B,C,D,F,E,O
B:A,C,E,K

3、解题思路

A-B
A-C
...

A-B

A-B
B-A

A-B
A-B

A-B

A-E

A-E

E-A

A-E

package com.ghgj.mazh.mapreduce.exercise.fans;

import java.io.IOException;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.FileSystem;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.NullWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.Mapper;
import org.apache.hadoop.mapreduce.Reducer;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;

/**
 * 作者： 马中华：http://blog.csdn.net/zhongqi2513
 * 日期： 2017年8月6日下午3:42:57
 * 
 * 描述：求互为好友的好友对
 */
public class FansMR {

	public static void main(String[] args) throws Exception {
		// 指定hdfs相关的参数
		Configuration conf = new Configuration();
//		conf.set("fs.defaultFS", "hdfs://hadoop02:9000");
//		System.setProperty("HADOOP_USER_NAME", "hadoop");

		Job job = Job.getInstance(conf);
		// 设置jar包所在路径
		job.setJarByClass(FansMR.class);

		// 指定mapper类和reducer类
		job.setMapperClass(FansMRMapper.class);
		job.setReducerClass(FansMRReducer.class);

		// 指定maptask的输出类型
		job.setMapOutputKeyClass(Text.class);
		job.setMapOutputValueClass(Text.class);
		// 指定reducetask的输出类型
		job.setOutputKeyClass(Text.class);
		job.setOutputValueClass(NullWritable.class);

		// 指定该mapreduce程序数据的输入和输出路径
		Path inputPath = new Path("D:\\bigdata\\commonfriends\\input");
		Path outputPath = new Path("D:\\bigdata\\commonfriends\\hufen_output");
		FileSystem fs = FileSystem.get(conf);
		if (fs.exists(outputPath)) {
			fs.delete(outputPath, true);
		}
		FileInputFormat.setInputPaths(job, inputPath);
		FileOutputFormat.setOutputPath(job, outputPath);

		// 最后提交任务
		boolean waitForCompletion = job.waitForCompletion(true);
		System.exit(waitForCompletion ? 0 : 1);
	}

	private static class FansMRMapper extends Mapper<LongWritable, Text, Text, Text> {
		@Override
		protected void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException {
			String line = value.toString();
			String[] tokens = line.split(":");
			String person = tokens[0];
			String[] fens = tokens[1].split(",");

			/**
			 * 写出的数据格式：
			 * AB ---> A
			 * AB ---> B
			 * 如果AB是互粉对，那么必然 AB 所对应的的 values组合会有两个值，也就是必定会有 A 和 B
			 */
			for (String fen : fens) {
				context.write(new Text(combineStr(person, fen)), new Text(person));
			}
		}

		/**
		 * 此方法的作用就是把 AB 和 BA 的组合 都编程 AB
		 */
		private String combineStr(String a, String b) {
			if (a.compareTo(b) > 0) {
				return b +"-"+ a;
			} else {
				return a +"-"+ b;
			}
		}
	}

	private static class FansMRReducer extends Reducer<Text, Text, Text, NullWritable> {
		@Override
		protected void reduce(Text key, Iterable<Text> values, Context context) throws IOException, InterruptedException {
			int sum = 0;
			for (Text t : values) {
				sum++;
			}
			if (sum == 2) {
				context.write(key, NullWritable.get());
			}
		}
	}
}

5、结果

A-B
A-C
A-D
A-F
A-O
B-E
C-F
D-E
D-F
D-L
E-L
E-M
F-M
H-O
I-O
J-O