title: a better guess on upper bound
body: <p>It's a question from 《introduction to algorithms》whose number is 4.4-5 and is described like this:</p>
  
  <blockquote>
    <p>Use a recursion tree to determine a good asymptotic upper bound on the recurrence T(n) = T(n-1) + T(n/2) + n.Use the substitution method to verify your answer.</p>
  </blockquote>
  
  <p>I found it is difficult to me to calculate the recursion tree's recurrence. The answer I gave </p>
  
  <blockquote>
    <p>Math.pow(2,n)</p>
  </blockquote>
  
  <p>seems too loose.Maybe there is some better guess existed.Thanks for any help.</p>