【笔试】两个大数相乘

package com.baidu.ecom;


//import java.math.BigInteger;
import java.util.Scanner;
public class BigMultiTest {

public static void main(String[] args) 
	{
		Scanner reader=new Scanner(System.in);
		String numStr1=reader.next();
		String numStr2=reader.next();
	
		int[] intNum1=changetoArray(numStr1);
		int[] intNum2=changetoArray(numStr2);
//		BigInteger bi = new BigInteger();
		multiply(intNum1,intNum2);
	}

public static int[] changetoArray(String numStr)
	{
		int length=numStr.length();
		int[] intNum=new int[length];
		for(int i=0;i<length;i++)
		intNum[length-i-1]=Integer.parseInt(String.valueOf((numStr.charAt(i))));
		return intNum;
	}

public static int[] multiply(int[] num1,int[] num2)
	{
		int length1=num1.length;
		int length2=num2.length;
		int[] result=new int[length1+length2];
		for(int i=0;i<length1;i++)//进行相乘运算
			for(int j=0;j<length2;j++)
			{
				int temp = result[i + j] + num1[i] * num2[j];
				result[i + j] = temp % 10;
				result[i + j + 1] += temp / 10;
				if (result[i + j + 1] > 10) {
					result[i + j + 1] %= 10;
					result[i + j + 2]++;
				}
			}	
		StringBuffer sb=new StringBuffer();
		boolean flag = false;//结合result[i]==0来清除前面多与的0
		for(int i=result.length-1;i>=0;i--)//通过大数数组来构造字符串
		{
			if(result[i]==0&&flag==false)
					continue;
			flag = true;//只要出现了以一个不为0的数字，就置flag为true。这句话放的位置不好，可以改进下
			sb.append(result[i]);
		}
		System.out.println(sb.toString());//输出大数字符串
		return result;
	}

}

 结果：

 

123456789
987654321
121932631112635269

123456
654321
8779853376

45600
5689056
259420953600