title: Why is the complexity of computing the Fibonacci series 2^n and not n^2?
body: <p>I am trying to find complexity of Fibonacci series using a recursion tree and concluded <code>height of tree = O(n)</code> worst case, <code>cost of each level = cn</code>, hence <code>complexity = n*n=n^2</code></p>
  
  <p>How come it is <code>O(2^n)</code>?</p>