import java.util.*;
import java.util.TreeMap;
import java.util.Set;
import java.util.Map;
import java.util.Iterator;

public class PhoneBook {
    String phoneNumber;
    Map<String, String> pb = new TreeMap <String, String>();

    public PhoneBook(){
    }

    public void add(String name, String phoneNumber){

        pb.put(name, phoneNumber);

    }

    public void remove(String name){
        pb.remove(name);
    }

    public String display(){
        Set set = pb.entrySet();
        Iterator it = set.iterator();
        StringBuilder display = new StringBuilder();

        while(it.hasNext()) {
            Map.Entry me = (Map.Entry)it.next();
            display.append(me.getKey() + " : " + me.getValue() + "\n");
            
        }
        return display.toString();
    }

    public String lookup(String name){
        String phone = pb.get(name);
        return phone;
    }

    public String reverseLookup(String phoneNumber){
        String key= null;
        for(Map.Entry entry: pb.entrySet()){
            if(phoneNumber.equals(entry.getValue())){
                key = entry.getKey().toString();
                break; 
            }
        }
        return key;

    }

    public int size() {
        return pb.size();
    }
}