Java Fuck Bignumber

　为了熟悉java , 开一套poj大数处理的题来写.

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A:

　　（1）主要时间花在在处理输入上:

　　System.setIn(new FileInputStream("test.txt"));

　　相当于C++ 里面的　　　freopen("test.txt","r",stdin);

　　System.setOut(new FileOutputStream("ans.out"));

　　相当于C++里面的　　　 freopen("ans.out","w",stdout);

　　它应该要加在　　Scannercin = new Scanner (System.in); 的前面;

　　（2）cin.hashNext()  用来判断读入结束;

　　

　　文件输入输出:

　　BufferedReader rd = new BufferedReader(new FileReader("lottery.in"));

　　PrintWriter ot = new PrintWriter("lottery.out");

　　Scanner in = new Scanner(rd);

　　ot.println(ansBigInteger);

　　rd.close();

　　ot.close();

import java.io.*;
import java.math.*;
import java.util.Scanner;

public class Main
{
    public static void main(String[] args) throws FileNotFoundException
    {
    //    System.setIn(new FileInputStream("test.txt"));
        Scanner cin = new Scanner (System.in);

        String str;
        BigDecimal ans,cur,eight;

        while (cin.hasNext())
        {
            eight = BigDecimal.valueOf(8);
            ans = BigDecimal.valueOf(0);
            cur = BigDecimal.valueOf(1);

            str = cin.nextLine();
            int pos = str.indexOf(".");
            BigDecimal tmp;
            for (int i = pos+1 ; i < str.length(); i++)
            {
                cur = cur.divide(eight);
                tmp = BigDecimal.valueOf(str.charAt(i)-'0');

                tmp = tmp.multiply(cur);
                ans = ans.add(tmp);
            }
            System.out.println(str+" [8]"+" = "+ans+" [10]");
        }
        System.exit(0);
    }
}

B：

　　（1）void main要设成 static ， 于是所有要调用的函数都要用static修饰;

　　（2）BigIntegerans ans = BigInteger.valueOf(0);　　//可以得到一个值为int的 大数 ans

　　（3）大数ans的加/减/乘/除/都可以通过调用 ans.add(t) / add.subtract(t) / ans.multiply(t) /ans.divide(t) ； 参数t 必须也是大数;

 import java.io.FileInputStream;
 import java.io.FileNotFoundException;
 import java.math.BigInteger;
 import java.util.Arrays;
 import java.util.Scanner;

 public class Main
 {
     static private int maxn = 110;
     static BigInteger dp[] = new BigInteger[maxn];
     static BigInteger unvis = BigInteger.valueOf(-1);

     static private BigInteger getVal(int n)
     {
         BigInteger ans;
         ans = BigInteger.valueOf(n);
         /*
         for (int i=1 ; i<=n ; i++ )
             ans = ans.multiply( BigInteger.valueOf(2) );
         ans = ans.subtract(BigInteger.valueOf(1));*/
         return ans;
     }
     static private BigInteger solv(int n)
     {
         BigInteger ans = BigInteger.valueOf(0);
         BigInteger tmp;
         int i;
         if (dp[n] != unvis) return dp[n];
     //    System.out.println("n="+n);
         for ( i=1 ; i<=n ; i++ )
         {
             tmp = getVal(i);
             tmp = tmp.multiply(solv(n-i));
     //        System.out.println("i="+i+" tmp="+tmp);
             ans = ans.add(tmp);
         }
         return dp[n] = ans;
     }

     static public void main(String[] args) throws FileNotFoundException
     {
         Scanner cin = new Scanner(System.in);
         int n;
         while (cin.hasNext())
         {
             n = cin.nextInt();
             Arrays.fill(dp, unvis);
             dp[0] = BigInteger.valueOf(1);
             System.out.println(solv(n));
         }
         System.exit(0);
     }

 }

C:

　　（1）大数的进制转换:

　　 BigInter m = new BigInteger(st, base); // st以base进制转换成10进制

　　 类似的进制转换还有：

　 　String str = Integer.toString(num, base); // 把num当做10进制的数转成base进制的str(base <= 35).

　　 int num = Integer.parseInt(st, base); // 把st当做base进制，转成10进制的int

　　（2）关于字符串的操作:

　　 java的string是不可变类型,需要进行各种修改赋值操作的话,还是用stringBuffer比较方便

　　 相关资料

 import java.io.FileInputStream;
 import java.io.FileNotFoundException;
 import java.math.BigInteger;
 import java.util.Scanner;

 public class Main
 {
     static BigInteger trans[] = new BigInteger[1000];
     static char retrans[] = new char[1000];
     static String TransToString(BigInteger ans,BigInteger base)
     {
         StringBuffer res = new StringBuffer();
         BigInteger zero = BigInteger.valueOf(0);
         if (ans.compareTo(zero) == 0)
         {
             res = res.append(0);
             return res.toString();
         }
         while (ans.compareTo(zero) > 0)
         {
             int tmp = ans.mod(base).intValue();
             res = res.append(retrans[tmp]);
             ans = ans.divide(base);
         }
         res = res.reverse();
         return res.toString();
     }
     static String getAns(String str,BigInteger base1,BigInteger base2)
     {
         int len = str.length(),i;
         char[] ch = str.toCharArray();
         BigInteger cur = BigInteger.valueOf(1);
         BigInteger ans = BigInteger.valueOf(0);
         for ( i=len-1 ; i>=1 ; i-- )
         {
             BigInteger tmp = trans[ch[i]].multiply(cur);
             cur = cur.multiply(base1);
             ans = ans.add(tmp);
         }
         String resString = TransToString(ans,base2);
         return resString;
     }
     static void initTrans()
     {
         int idx=0;
         char i;
         for ( i='0' ; i<='9' ; i++ )    {trans[i] = BigInteger.valueOf(idx++);    retrans[idx-1]=i;}
         for ( i='A' ; i<='Z' ; i++ )    {trans[i] = BigInteger.valueOf(idx++);    retrans[idx-1]=i;}
         for ( i='a' ; i<='z' ; i++ )    {trans[i] = BigInteger.valueOf(idx++);    retrans[idx-1]=i;}
     }
     static public void main(String[] argStrings) throws FileNotFoundException
     {
 //        System.setIn(new FileInputStream("test.txt"));
         Scanner cin = new Scanner(System.in);
         String str;
         initTrans();
         int cas;
         BigInteger base1,base2;
         String ans;
         cas = cin.nextInt();
         for (int i = 0; i < cas; i++)
         {
             base1 = cin.nextBigInteger();
             base2 = cin.nextBigInteger();
             str = cin.nextLine();
             ans = getAns(str, base1, base2);
             System.out.println(base1+str);
             System.out.println(base2+" "+ans);
             System.out.println();
         }
         System.exit(0);
     }
 }

　　JAVA中的重定向: