title: Calculate value of n choose k
body: <p>What is the most efficient method to evaluate the value of n choose k ? 
  The brute force way I think would be to find n factorial / k factorial / (n-k) factorial . </p>
  
  <p>A better strategy may be to use dp according to this <a href="http://i.stack.imgur.com/Kq3OH.png">recursive formula</a>. Is there any other better method to evaluate n choose k ? </p>