大数相加、大数相乘

我们首先看大数相加，其基本思路是：

       过程和我们直接笔算思路一样，比如  129 + 23 ，先计算 9+3 = 12 ,则确定个位为2，同时向前进 1 ；继续 2 + 2 + 1 = 5，确定十位为5，同时向前进 0；继续 1 + 0 + 0 = 1,确定百位为1（其中该式中前一个0为12补齐的0，后一个0为进位的0）；结束 得到结果为152。代码如下：

package pack1;public class Multiply {/** * @param 大数相加 */public String add(String str1,String str2){int jinwei = 0;StringBuffer sb1 = new StringBuffer(str1); sb1.reverse();StringBuffer sb2 = new StringBuffer(str2); sb2.reverse();int len1=sb1.length(),len2=sb2.length(),minLen = (len1>len2)?len2:len1;StringBuffer sb = new StringBuffer("");for(int i=0;i<minLen;i++){int single =Integer.parseInt(sb1.charAt(i)+"")+Integer.parseInt(sb2.charAt(i)+"")+jinwei;sb.append(single%10);jinwei = single /10;}if(len1==len2){if(jinwei!=0)  sb.append(jinwei);return sb.reverse().toString();}else if(len1>len2){for(int i=len2;i<len1;i++){int single =Integer.parseInt(sb1.charAt(i)+"")+jinwei;sb.append(single%10);jinwei = single /10;}if(jinwei!=0) sb.append(jinwei);return sb.reverse().toString();}else{for(int i=len1;i<len2;i++){int single =Integer.parseInt(sb2.charAt(i)+"")+jinwei;sb.append(single%10);jinwei = single /10;}if(jinwei!=0) sb.append(jinwei);return sb.reverse().toString();}}        public static void main(String[] args) {Multiply mul = new Multiply();System.out.println(mul.add("8882488", "123888234248888"));//使用BigInteger               java.math.BigInteger  big1 = new java.math.BigInteger("8882488");               java.math.BigInteger  big2 = new java.math.BigInteger("123888234248888");               System.out.println(big1.add(big2).toString());}}

大数相乘

大数相乘怎么做呢，比如计算129 * 23，算法思路为：将乘法变换为加法进行计算。首先取23的个位3，代表129需要相加3次；然后再取23的十位2，代表129需要相加2次，并且同时需要在乘以10，则可以得到最后的结果。代码如下：

package pack1;public class Multiply {/** * @param 大数相加 */public String add(String str1,String str2){int jinwei = 0;StringBuffer sb1 = new StringBuffer(str1); sb1.reverse();StringBuffer sb2 = new StringBuffer(str2); sb2.reverse();int len1=sb1.length(),len2=sb2.length(),minLen = (len1>len2)?len2:len1;StringBuffer sb = new StringBuffer("");for(int i=0;i<minLen;i++){int single =Integer.parseInt(sb1.charAt(i)+"")+Integer.parseInt(sb2.charAt(i)+"")+jinwei;sb.append(single%10);jinwei = single /10;}if(len1==len2){if(jinwei!=0)  sb.append(jinwei);return sb.reverse().toString();}else if(len1>len2){for(int i=len2;i<len1;i++){int single =Integer.parseInt(sb1.charAt(i)+"")+jinwei;sb.append(single%10);jinwei = single /10;}if(jinwei!=0) sb.append(jinwei);return sb.reverse().toString();}else{for(int i=len1;i<len2;i++){int single =Integer.parseInt(sb2.charAt(i)+"")+jinwei;sb.append(single%10);jinwei = single /10;}if(jinwei!=0) sb.append(jinwei);return sb.reverse().toString();}}    public String multiply(String str1,String str2){    int len1 = str1.length();    int len2 = str2.length();    String result = "0";    if(len1<len2){              //使得str2指向的字符串为较短的一个字符串    String  temp = str1;    str1 = str2;    str2 = temp;    len1 = str1.length();        len2 = str2.length();    }              for(int i=1;i<=len2;i++){    String temp = "0";    int num = str2.charAt(len2-i) - '0';    for(int j=1;j<=num;j++)    temp = add(temp,str1);                for(int k=1;k<=i-1;k++)    temp = temp + "0";        result = add(result,temp);    }        return result;    }public static void main(String[] args) {Multiply mul = new Multiply();System.out.println(mul.multiply("8882488", "123888234248888"));//使用BigInteger        java.math.BigInteger  big1 = new java.math.BigInteger("8882488");        java.math.BigInteger  big2 = new java.math.BigInteger("123888234248888");        System.out.println(big1.multiply(big2).toString());}}