Project Euler > Problem 25 > 1000-digit Fibonacci number (Java Solution)

Problem:

The Fibonacci sequence is defined by the recurrence relation:

Fn = Fn[−]1 + Fn[−]2, where F1 = 1 and F2 = 1.

Hence the first 12 terms will be:

F1 = 1
F2 = 1
F3 = 2
F4 = 3
F5 = 5
F6 = 8
F7 = 13
F8 = 21
F9 = 34
F10 = 55
F11 = 89
F12 = 144

The 12th term, F12, is the first term to contain three digits.

What is the first term in the Fibonacci sequence to contain 1000 digits?

Solution:

4782

Code:
The solution may include methods that will be found here: Library.java .

public interface EulerSolution{

public String run();

}

/* 
 * Solution to Project Euler problem 25
 * By Nayuki Minase
 * 
 * http://nayuki.eigenstate.org/page/project-euler-solutions
 * https://github.com/nayuki/Project-Euler-solutions
 */

import java.math.BigInteger;


public final class p025 implements EulerSolution {
 
 public static void main(String[] args) {
  System.out.println(new p025().run());
 }
 
 
 private static final int DIGITS = 1000;
 
 public String run() {
  BigInteger lowerthres = BigInteger.TEN.pow(DIGITS - 1);
  BigInteger upperthres = BigInteger.TEN.pow(DIGITS);
  BigInteger prev = BigInteger.ONE;
  BigInteger cur = BigInteger.ZERO;
  int i = 0;
  while (true) {
   // At this point, prev = fibonacci(i - 1) and cur = fibonacci(i)
   if (cur.compareTo(lowerthres) >= 0)
    return Integer.toString(i);
   else if (cur.compareTo(upperthres) >= 0)
    throw new RuntimeException("Not found");
   
   BigInteger temp = cur.add(prev);
   prev = cur;
   cur = temp;
   i++;
  }
 }
 
}