title: Compact array in C
body: <p>Suppose i have these pairs of nodes (ID):</p>
  
  <ul>
  <li>0   1 </li>
  <li>0   8</li>
  <li>500 4</li>
  <li>8   300</li>
  </ul>
  
  <p>I know the number of unique nodes: 6.
  How can i save them nto a matrix, without allocating a 500x500 matrix?
  Which kind of mapping can be used?</p>
  
  <p>Thanks in advance!</p>