求解5000阶乘之和

import java.math.   ;
import java.util.Date;
import java.util.HashMap;
public class Teest {
	public static void main(String[] args) {
		long start = (new Date()).getTime();
		BigDecimal res = BigDecimal.ONE;
		for (int i = 2; i <= 5000; i++) {
			res = multiply(res,BigDecimal.valueOf(i));
		}
		int sum=0;
		String str = res.toPlainString().replace("0","");

		for (int i = 0; i < str.length(); i++) {
			sum += Character.getNumericValue(str.charAt(i));
		}
		long end = (new Date()).getTime();
		System.out.println("花费的时间："+(end-start) +"ms");
		
		System.out.println("5000阶乘结果之和为："+sum);

	}
	//两个数相乘,科学计算法表示
	public static BigDecimal multiply(BigDecimal b1,BigDecimal b2){
		return b1.multiply(b2).stripTrailingZeros();
	}
}

	private static class Factorial extends RecursiveTask<BigDecimal>{
		private final static long serialVersionUID = 1L;
		int start;
		int end;
		Factorial(int start,int end){
			this.start = start;
			this.end = end;
		}
		@Override
		protected BigDecimal compute() {
			BigDecimal res = BigDecimal.ONE;
			if(end - start < 100){
				for (int i = start; i <= end; i++) {
					res = multiply(res,BigDecimal.valueOf(i));
				}
			}else {
				int mid = (start + end) / 2;
				Factorial left = new Factorial(start,mid);
				Factorial right = new Factorial(mid+1 , end);
				left.fork();
				right.fork();
				res = multiply(left.join() , right.join());
			}
			return res;
		}
	}
	public static void main(String[] args) throws ExecutionException, InterruptedException {
		long start = (new Date()).getTime();
		Factorial facTask = new Factorial(1,5000);
		ForkJoinPool forkJoinPool = new ForkJoinPool();
		Future<BigDecimal> res = forkJoinPool.submit(facTask);

		String str = res.get().toPlainString().replace("0","");
//		int sum = 0;
//		for (int i = 0; i < str.length(); i++) {
//			sum += Character.getNumericValue(str.charAt(i));
//		} 
		Sum sumtask = new Sum(0,str.length(),str);
		int sum =  forkJoinPool.submit(sumtask).get();
		long end = (new Date()).getTime();
		System.out.println("花费的时间："+(end - start) + "ms");
		System.out.println("5000阶乘之和："+sum);

	}
	private static class Sum extends RecursiveTask<Integer>{
		private final static long serialVersionUID = 1L;
		int start;
		int end;
		String str;
		Sum(int start,int end,String str){
			this.start = start;
			this.end = end;
			this.str = str;
		}
		@Override
		protected Integer compute() {
			int sum = 0;
			if(end - start < 5000){
				for (int i = start; i < end; i++) {
					sum += Character.getNumericValue(str.charAt(i));
				}
			}else {
				int mid = (start + end) / 2;
				Sum left = new Sum(start,mid,str);
				Sum right = new Sum(mid , end,str);
				left.fork();
				right.fork();
				sum = left.join() + right.join();
			}
			return sum;
		}
	}
	//两个数相乘,科学计算法表示
	public static BigDecimal multiply(BigDecimal b1,BigDecimal b2){
		return b1.multiply(b2).stripTrailingZeros();
	}