fibonacci.java

public static BigInteger fibonacci(long n) {
  if (n < 1) throw new IllegalArgumentException("n>=1 must hold");
  BigInteger cur = BigInteger.ONE, t1 = BigInteger.ONE, t2 = BigInteger.ONE;
  for (long i = 2; i < n; i++) {
    cur = t1.add(t2);
    t1 = t2;
    t2 = cur;
  }
  return cur;
}