private static int prefixLen(String o1,String o2){
  final int l1=o1.length(), l2=o2.length(), l=l1 < l2 ? l1 : l2;
  int prefix=0;
  while (prefix < l && (o1.charAt(prefix) == o2.charAt(prefix))) {
    prefix++;
  }
  return prefix;
}

private static int prefixLen ( String o1 , String o2 ) { final int l1 = o1 . length ( ) , l2 = o2 . length ( ) , l = l1 < l2 ? l1 : l2 ; int prefix = 0 ; while ( prefix < l && ( o1 . charAt ( prefix ) == o2 . charAt ( prefix ) ) ) { prefix ++ ; } return prefix ; }
compute the length of the prefix .
