大数乘法：
public class BigNumberMulti {
public static void main(String[] args) {
String num1Str = "24566";
String num2Str = "452053";
int[] num1 = new int[num1Str.length()];
int[] num2 = new int[num2Str.length()];
//将字符串转换为整数数组，注意，24566应转化成{6,6,5,4,2}
for(int i = 0; i < num1Str.length();i++) {
num1[num1Str.length() -1- i] = num1Str.charAt(i)-'0';
}
for(int i = 0; i < num2Str.length();i++) {
num2[num2Str.length() -1- i] = num2Str.charAt(i)-'0';
}
int[] result = multiply(num1, num2);
for(int i = result.length-1; i>=0;i--){
System.out.print(result[i]);
}
}
public static int[] multiply(int[] num1, int[] num2) {
int lengthOfNum1 = num1.length;
int lengthOfNum2 = num2.length;
// 分配一个空间，用来存储运算的结果，n1长的数* n2长的数，结果不会超过n1+n2长
int[] result = new int[lengthOfNum1 + lengthOfNum2];
// 先不考虑进位问题，根据小学竖式的乘法运算，n1的第i位与n2的第j位相乘，结果应该存放在结果的第i+j位上
for (int i = 0; i < lengthOfNum1; i++) {
for (int j = 0; j < lengthOfNum2; j++) {
result[i + j] += num1[i] * num2[j];
}
}
// 单独处理进位问题
for (int i = 0; i < lengthOfNum1 + lengthOfNum2 -1; i++) {
if (result[i] > 10) {
result[i + 1] += result[i] / 10;
result[i] %= 10;
}
}
return result;
}
}
