title: Adjacency Matrix Implementation
body: <p>I'm looking for an implementation of a Matrix with keys at first row and first column.
  First I thought something like a HashMap in a HashMap but this looks ugly and I think it would be wrong.
  I really need those keys where I can check, if a string exists in the first row as a key.
  (I want  to implement an adjacency matrix)</p>
  
  <p>I'm not sure if I was clear enough. Here a small Picture to visualize it.</p>
  
  <p>Something like this: </p>
  
  <p><img src="https://www.unterstufe.ch/angebote/bsp_pics/m-2kl-1x1_tabelle_kleines_1x1_04_matrix-ID_12673.gif" alt=""></p>
  
  <p>Just that the first column and row are not numbers but cities.</p>
  
  <p>In fact I want to save the distances from cities to cities in a adjacency matrix.</p>