Reverse many-to-many Dictionary>

Question

Actually my previous question got me thinking and I realized that reversing a Dictionary is not trivial. What is the most elegant and readable way to do it?

Answer 1

Slightly different way (a bit more comprehensible to my brain anyway :) ...

var newDict = new Dictionary>();
var dict = new Dictionary>();
dict.Add( 1, new List() { 1, 2, 3, 4, 5 } );
dict.Add( 2, new List() { 1, 2, 3, 4, 5 } );
dict.Add( 3, new List() { 1, 2, 6 } );
dict.Add( 4, new List() { 1, 6, 7 } );
dict.Add( 5, new List() { 8 } );

var newKeys = dict.Values.SelectMany( v => v ).Distinct();

foreach( var nk in newKeys )
{
   var vals = dict.Keys.Where( k => dict[k].Contains(nk) );
   newDict.Add( nk, vals.ToList() );
}