need algorith to combine sets

[Paul]

Hi I have to lists, using C# and the look like
List <string> reportname = new List<string>();
List <Int32> reportnumber = new List <Int32>();
They get filled with corresponding data, both lists are the same size. I
need to create a third string lists and fourth integer list that combines the
report names when the report numbers are the same. For example the data
might look like
reportname report number
vehicles 1
bikes 1
cities 2

so I would like to create a third and fourth list that would have
report name report number
vehicles, bikes 1
cities 2
thanks.