SumTime.java

/**
 * Created by olegchir on 18.11.2016.
 */
import java.math.BigInteger;
import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.RecursiveTask;

class Sum extends RecursiveTask<BigInteger> {
    private static final BigInteger MAX = new BigInteger("1000000000");
    private static final BigInteger STREAK = MAX.divide(new BigInteger("16"));
    private static final BigInteger TWO = new BigInteger("2");

    BigInteger low;
    BigInteger high;

    public static void main(String[] args) {
        long startTime = System.currentTimeMillis();
        BigInteger result = Sum.run();
        long endTime = System.currentTimeMillis();
        System.out.println(result.toString());
        System.out.println("---");
        System.out.println(endTime - startTime);
    }

    Sum(BigInteger lo, BigInteger hi) {
        low   = lo;
        high  = hi;
    }

    protected BigInteger compute() {
        if(high.subtract(low).compareTo(STREAK) <= 0) {
            BigInteger sum = BigInteger.ZERO;
            BigInteger i = low;
            while(true) {
                i = i.add(BigInteger.ONE);
                if (i.compareTo(high) > 0) {
                    break;
                } else {
                    sum = sum.add(i);
                }
            }
            return sum;
        } else {
            BigInteger mid = low.add(high.subtract(low).divide(TWO));
            Sum left  = new Sum(low, mid);
            Sum right = new Sum(mid, high);
            left.fork();
            BigInteger rightAns = right.compute();
            BigInteger leftAns  = left.join();
            return leftAns.add(rightAns);
        }
    }

    static BigInteger run() {
        return ForkJoinPool.commonPool().invoke(new Sum(BigInteger.ZERO, MAX));
    }
}