Algorithms—43.Multiply Strings

public class Solution {
public String multiply(String num1, String num2) {
if (num1.equals("0")||num2.equals("0")) {
return "0";
}
if (num1.length()<num2.length()) {
return multiply(num2,num1);
}
int l1=num1.length();
int[] n1=new int[l1];
for (int i = 0; i <l1; i++) {
n1[l1-i-1]=Integer.parseInt(num1.substring(i, i+1));
}
int l2=num2.length();
int[] n2=new int[l1];
for (int i = 0; i <l2; i++) {
n2[l2-i-1]=Integer.parseInt(num2.substring(i, i+1));
}
int[] a=new int[l1+l2-1];
int sum=0;
int carry=0;
for (int i = 0; i < a.length; i++) {
// 每一位乘级相加的结果
for (int j = Math.max(0,i-l2+1); j <= Math.min(i,l1-1); j++) {
sum += n1[j] * n2[i-j];
}
// 加进位
sum += carry;
carry = sum / 10;
sum%=10;
a[i]=sum;
sum = 0;
}
StringBuffer sb=new StringBuffer();
if (carry!=0) {
sb.append(String.valueOf(carry));
}
for (int i = a.length-1; i >=0 ; i--) {
sb.append(String.valueOf(a[i]));
}
return sb.toString();
}
}