title: C - converting to 2s complement
body: <p>I've decided to do it this way</p>
  
  <ul>
  <li>flip numbers 0=1, 1=0</li>
  <li>add 1 to LSB</li>
  <li>if carry, loop until array[i]==0</li>
  </ul>
  
  <p>But I'm stuck on the last point; how can I say that in a conditional loop?</p>