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  3. Project Euler #25: N-digit Fibonacci number
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Project Euler #25: N-digit Fibonacci number

  • v33vikasgautam
     + 0 comments

    here is a true O(1) solution. No dp, no loops nothing, Just reverse engineering.

    ; Takes the length of fib return the (index of fib) + 1
(defn fib-len-rev [l]
  (let [phi (-> (Math/sqrt 5) (+ 1) (/ 2))]
  (-> (+ (dec l)
         (* 1/2 (Math/log10 5)))
      (/ (Math/log10 phi))
      (Math/ceil)
      (int))))