字符串相乘

char* multiply(char* num1, char* num2) {
    if(*num1=='0'||*num2=='0')return "0";
	int len1=strlen(num1),len2=strlen(num2);
	char*res=(char*)malloc((len1+len2+1)*sizeof(char));
	for(int i=0; i<len1+len2; i++)res[i]=48;
    res[len1+len2]='\0';
	for(int i=0; i<len2; i++) {
		for(int j=0; j<len1; j++) {		    
            int result=(num2[i]-'0')*(num1[j]-'0'),carryout=0;
            for(int b=i+j+1; b>=0; b--) {
                int h=0;
                if(b==i+j+1)h=result%10;
                else if(b==i+j&&(result)>9)h=result/10;
                else h=0;
                int tt=res[b]-'0'+h+carryout;
                res[b]=tt%10+'0';			
                carryout=tt/10;			
                if(!carryout&&b<=i+j)break;
            }
        }
	}
	return *res=='0'?++res:res;
}