Multiply Two Big Integers

http://www.zrzahid.com/production-of-big-integers/

//O(nm)
public static String prod(String str1, String str2){
 String res = new String("0");
 
 int count = 0;
 for(int i = str2.length()-1; i>=0 ; i--){
  int d2 = str2.charAt(i)-'0';
  
  int carry = 0;
  StringBuffer prod = new StringBuffer();
  for(int j = str1.length()-1; j>=0; j--){
   int d1 = str1.charAt(j)-'0';
   int p = carry+(d1*d2);
   prod.append(p%10);
   carry = p/10;
  }
  
  if(carry != 0){
   prod.append(carry);
  }
  
  prod.reverse();

  for(int k = 0; k<count; k++){
   prod.append(0);
  }
  
  res = add(res, prod.toString());
  count++;
 }
 
 return res.toString();
}

//O(n);
private static String add(String str1, String str2){
 StringBuffer res = new StringBuffer();
 
 int i = str1.length()-1;
 int j = str2.length()-1;
 int carry = 0;
 while(true){
  if(i < 0 && j < 0){
   break;
  }
  
  int d1 = i < 0 ? 0 : str1.charAt(i--)-'0';
  int d2 = j < 0 ? 0 : str2.charAt(j--)-'0';
  int sum = d1+d2+carry;
  
  res.append(sum%10);
  carry = sum/10;
 }
 
 if(carry != 0){
  res.append(carry);
 }
 
 return res.reverse().toString();
}