title: Finding the index of a reordered vector in a reordered large matrix
body: <p>Suppose I have a reordered matrix <code>A</code>, which is get by using <code>A = nchoosek(1:100, 6)</code> in MATLAB. Thus, <code>A</code> means all the combinations by selecting 6 digits from 1 to 100. </p>
  
  <p><code>A</code> is reordered, and the dim of <code>A</code> is very large (1e9*6), so <code>A</code> is like this: </p>
  
  <pre><code> 1     2     3     4     5     6
   1     2     3     4     5     7
   1     2     3     4     5     8
   .     .     .     .     .     .
   .     .     .     .     .     .
   94    96    97    98    99   100
   95    96    97    98    99   100
  </code></pre>
  
  <p>Then, I have another reordered vector <code>B</code>, which is a member of <code>A</code>. Such as <code>B=[5 10 11 40 51 67];</code></p>
  
  <p>So, how to find the index of <code>B</code> using the fastest way, which means utilizing the ordering information??</p>