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?

Same scenario student Many to Many with Classes

original Dicitonary<int, List<int>> where the key is studentId and the Value is a List<int> that contains classId and want to revert to Dictionary<classId, List<studentId>>

Thanks

Update:
Actually I just tested Luke and Bruno’s solutions and they return the right amount of classed whoever they all have the same student, will update as i go along.

Answer 1

Slightly different way (a bit more comprehensible to my brain anyway 🙂 …

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