求解代码

publicStringmultiply(Stringnum1,Stringnum2){if("0".equals(num1)||"0".equals(num2)){return"0";}intlen1=num1.length();intlen2=num2.length();int[]res=newint[len1+len2];// 从后往前遍历for(inti=len1-1;i>=0;i--){intn1=num1.charAt(i)-'0';for(intj=len2-1;j>=0;j--){intn2=num2.charAt(j)-'0';intsum=n1*n2+res[i+j+1];// 加上原来的低位res[i+j+1]=sum%10;res[i+j]+=sum/10;}}StringBuildersb=newStringBuilder();for(intnum:res){if(!(sb.length()==0&&num==0)){sb.append(num);}}returnsb.length()==0?"0":sb.toString();}