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  1. /* package whatever; // don't place package name! */
  2.  
  3. import java.util.*;
  4. import java.lang.*;
  5. import java.io.*;
  6.  
  7. /* Name of the class has to be "Main" only if the class is public. */
  8. class Ideone
  9. {
  10. 	public static Map<String, List<String>> inverse(Map<String, String> m)
  11.     {
  12.         Map<String, List<String>> r = new HashMap<>();
  13.         for (String k : m.keySet())
  14.         {
  15.             String s = m.get(k);
  16.             if (r.get(s) == null)
  17.                 r.put(s, new ArrayList<>());
  18.             r.get(s).add(k);
  19.         }
  20.         return r;
  21.     }
  22. 	public static void main (String[] args) throws java.lang.Exception
  23. 	{
  24. 		Map<String, String> map = new HashMap<>();
  25.         map.put("Clyde", "blue");
  26.         map.put("Sarah", "blue");
  27.         map.put("Fred", "green");
  28.         map.put("Derp", "green");
  29.         map.put("Barney", "yellow");
  30.  
  31.         System.out.println("Original:\n{");
  32.         for (String key : map.keySet())
  33.             System.out.println("\t" + key + " : " + map.get(key));
  34.         System.out.println("}\n\nInverse:\n{");
  35.         Map<String, List<String>> results = inverse(map);
  36.  
  37.         for (String key : results.keySet())
  38.         {
  39.             System.out.println("\t" + key + " : " + results.get(key));
  40.         }
  41.         System.out.println("}");
  42. 	}
  43. }
Success #stdin #stdout 0.04s 711168KB
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Standard input is empty
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Original:
{
	Clyde : blue
	Barney : yellow
	Sarah : blue
	Fred : green
	Derp : green
}

Inverse:
{
	green : [Fred, Derp]
	blue : [Clyde, Sarah]
	yellow : [Barney]
}
https://ideone.com/AZO9Uk
language:
Java (HotSpot 12)
created:
7 years ago
visibility:
public

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