mapreduce '找共同朋友'，面试题

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mapred找共同朋友，数据格式如下：

[quote]
A B C D E F
B A C D E
C A B E
D A B E
E A B C D
F A
[/quote]

第一字母表示本人，其他是他的朋友，找出有共同朋友的人，和共同朋友是谁


答案如下：

import java.io.IOException;
import java.util.Set;
import java.util.StringTokenizer;
import java.util.TreeSet;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.Path;
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.Mapper.Context;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;
import org.apache.hadoop.util.GenericOptionsParser;

public class FindFriend {

          public static class ChangeMapper extends Mapper<Object, Text, Text, Text>{                      
                   @Override
                   public void map(Object key, Text value, Context context) throws IOException, InterruptedException {
                             StringTokenizer itr = new StringTokenizer(value.toString());
                                 Text owner = new Text();
                                 Set<String> set = new TreeSet<String>();
                             owner.set(itr.nextToken());
                             while (itr.hasMoreTokens()) {
                                     set.add(itr.nextToken());
                             }             
                             String[] friends = new String[set.size()];
                             friends = set.toArray(friends);

                             for(int i=0;i<friends.length;i++){
                                     for(int j=i+1;j<friends.length;j++){
                                             String outputkey = friends[i]+friends[j];
                                             context.write(new Text(outputkey),owner);
                                     }                                     
                             }
                   }
          }

          public static class FindReducer extends Reducer<Text,Text,Text,Text> {                          
                        public void reduce(Text key, Iterable<Text> values, 
                                        Context context) throws IOException, InterruptedException {
                                  String  commonfriends =""; 
                              for (Text val : values) {
                                  if(commonfriends == ""){
                                          commonfriends = val.toString();
                                  }else{
                                          commonfriends = commonfriends+":"+val.toString();
                                  }
                               }
                              context.write(key, new Text(commonfriends));                                
                        }                          
          }


        public static void main(String[] args) throws IOException,
        InterruptedException, ClassNotFoundException {

            Configuration conf = new Configuration();
            String[] otherArgs = new GenericOptionsParser(conf, args).getRemainingArgs();
            if (otherArgs.length < 2) {
              System.err.println("args error");
              System.exit(2);
            }
            Job job = new Job(conf, "word count");
            job.setJarByClass(FindFriend.class);
            job.setMapperClass(ChangeMapper.class);
            job.setCombinerClass(FindReducer.class);
            job.setReducerClass(FindReducer.class);
            job.setOutputKeyClass(Text.class);
            job.setOutputValueClass(Text.class);
            for (int i = 0; i < otherArgs.length - 1; ++i) {
              FileInputFormat.addInputPath(job, new Path(otherArgs[i]));
            }
            FileOutputFormat.setOutputPath(job,
              new Path(otherArgs[otherArgs.length - 1]));
            System.exit(job.waitForCompletion(true) ? 0 : 1);

        }

}

运行结果：

AB      E:C:D
AC      E:B
AD      B:E
AE      C:B:D
BC      A:E
BD      A:E
BE      C:D:A
BF      A
CD      E:A:B
CE      A:B
CF      A
DE      B:A
DF      A
EF      A

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