SQL Query re-write assistance requested

Question

I am needing to know the best way to go about re-writing a bad query at my workplace that needs to run at TOP performance due to very large amounts of data that will be run through it.
I have simplified the data and table to the following mock example:
ID SEQ_N IND
1 1 Y
1 2 N
2 1 N
2 2 N
3 1 N
I need a query that returns each ID, as well as the maximum SEQ_N for each ID. The tricky part is that if there is a "Y" in the indicator (IND) column, then use the maximum SEQ_N for those. If there is no "Y" indicator, then use the maximum SEQ_N for the "N" indicator column. So given the above example data, the query executed would return the following results:
ID MAX(SEQ_N)
1 1
2 2
3 1
I hope that makes sense.. The query we have running currently creates a string and gets the maximum from that. The inner part of the query looks like the following:
select id, max(decode(IND,'Y','1',0') || to_char(SEQ_N, '0000000009')) from table
And the outer query selects from this (using a substring and to_number) to get the sequence number back.. So we have a decode, to_char, substr, and to_number in order to get the job done - this is very expensive; there must be a better way!
Thanks in advance!

Answer 1

Or:
SQL> with t as (select 1 as id, 1 as seq_n, 'Y' ind from dual union all
  2    select 1 as id, 2 as seq_n, 'N' as ind from dual union all
  3    select 2 as id, 1 as seq_n, 'N' as ind from dual union all
  4    select 2 as id, 2 as seq_n, 'N' as ind from dual union all
  5    select 3 as id, 2 as seq_n, 'N'from dual)
  6  select id,max(seq_n) keep(dense_rank first order by ind desc,seq_n desc) seq_n
  7  from t
  8  group by id;
        ID      SEQ_N
         1          1
         2          2
         3          2