java大数类

java.math.BigInteger提供的方法：

1. BigInteger add(BigInteger other)
2. BigInteger subtract(BigInteger other)
3. BigInteger multiply(BigInteger other)
4. BigInteger divide(BigIntegr other)
5. int compareto(BigInteger other)
6. BigInteger mod(BigInteger other)

7. static BigInteger valueOf(long x)

   valueOf方法可以将普通的数值转换为大数值：

BigInteger a＝BigInteger.valueOf(100);
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输出febonacci数列

输出febonacci数列前100项，大约在93项时会超出long的范围：

public static void  fibonacci(){
        long[] a=new long[100];
        for(int i=0;i<100;i++){
            if(i==0){
                a[0]=0;
                System.out.print(a[i]+"\t");
                continue;
            }else if(i==1){
                a[1]=1;
                System.out.print(a[i]+"\t");
                continue;
            }
            a[i]=a[i-1]+a[i-2];
            System.out.print(a[i]+"\t");
            if((i+1)%5==0){System.out.println("");}
        }
    }
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改进：

public static void fibonacci() {
        BigInteger[] a = new BigInteger[200];
        for (int i = 0; i < 200; i++) {
            if (i == 0) {
                a[0] = BigInteger.valueOf(0);
                System.out.print(a[i] + "\t");
                continue;
            } else if (i == 1) {
                a[1] = BigInteger.valueOf(1);
                System.out.print(a[i] + "\t");
                continue;
            }
            a[i] = a[i - 1].add(a[i - 2]);
            System.out.println("第" + i + "项:\t" + a[i] + "\t");
        }
    }
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