title: How can I get all the unique keys in a multimap
body: <p>I have a multimap and I want get all the unique keys in it to be stored in a vector.</p>
  
  <pre><code>  multimap&lt;char,int&gt; mymm;
    multimap&lt;char,int&gt;::iterator it;
    char c;
  
    mymm.insert(pair&lt;char,int&gt;('x',50));
    mymm.insert(pair&lt;char,int&gt;('y',100));
    mymm.insert(pair&lt;char,int&gt;('y',150));
    mymm.insert(pair&lt;char,int&gt;('y',200));
    mymm.insert(pair&lt;char,int&gt;('z',250));
    mymm.insert(pair&lt;char,int&gt;('z',300));
  </code></pre>
  
  <p>How can I do this? there is way to count number of elements with a key but none to count number of unique keys in a multimap. </p>
  
  <p><strong>Added:</strong> By unique I mean all the keys in multimap once - they can be repeated or occur once in multimap.</p>
  
  <p>So unique keys here are - <strong>x</strong>, <strong>y</strong> and <strong>z</strong></p>