title: Big(O) of recursive n choose k code
body: <p>Is the big(O) of the following recursive code simply O(n choose k)?</p>
  
  <pre><code>int nchoosek(int n, int k) {
      if (n == k) return 1;
      if (k == 0) return 1;
      return nchoosek(n-1, k) + nchoosek(n-1, k-1);
  }
  </code></pre>