两个大数相乘JAVA版

import java.util.Scanner;

public class BigNumMul {

public static void main(String[] args) {
Scanner canner = new Scanner(System.in);
String str1 = canner.nextLine();
String str2 = canner.nextLine();
canner.close();
System.out.println(BigNumMul.mult(str1, str2));
}

/*
* str1 因子 str2 因子
*
* @result 乘积
*/
public static String mult(String str1, String str2) {
String result = "";
int[] rs = BigNumMul.mul(BigNumMul.strToNum(str1),
BigNumMul.strToNum(str2));
if (rs == null) {
System.out.println("输入错误");
} else {
result = BigNumMul.changeToString(rs);
}
return result;
}

/*
* num1 因子 num1[i] 对应数字num1.length-i+1位 str2 因子
*
* @result 乘积
*/

public static int[] mul(int[] num1, int[] num2) {
if (num1 == null || num1.length == 0 || num2 == null
|| num2.length == 0)
return null;
// 两数乘积位数不会超过乘数位数和+ 3位
int[] rs = new int[num1.length + num2.length + 3];
for (int i = 0; i < rs.length; i++) {
rs[i] = 0;
}
// 对齐逐位相乘
for (int i = 0; i < num2.length; i++) {
if (num2[i] == 0) {
continue;
}
for (int j = 0; j < num1.length; j++) {
if (num1[j] == 0) {
continue;
}
rs[j + i] += (num1[j] * num2[i]);
}
}
// 进位处理
for (int m = 0; m < rs.length; m++) {
int carry = rs[m] / 10;
rs[m] = rs[m] % 10;
if (carry > 0)
rs[m + 1] += carry;
}
return rs;
}

/*
* 将字符串转换成对应int数组
*/
public static int[] strToNum(String str) {
if (str == null || "".equals(str)) {
return null;
}
char[] c = str.toCharArray();
int[] num = new int[c.length];
// 高低位对调
for (int i = 0; i < c.length; i++) {
int m = c.length - i - 1;
if ('0' <= c[m] && c[m] <= '9') {
num[i] = Integer.parseInt(String.valueOf(c[m]));
System.out.println(num[i]);
} else {
return null;
}

}
System.out.println(BigNumMul.changeToString(num));
return num;
}

/*
* 将int数组转换成对应字符串
*/
public static String changeToString(int[] num) {
String rs = "";
int i = 0;
// 找到最高位
for (i = num.length - 1; i >= 0; i--) {
if (num[i] != 0)
break;
}
for (; i >= 0; i--) {
rs += num[i];
}
return rs;
}
}