## Solution

#### Approach: Union requester_id and accepter_id [Accepted]

Algorithm

Being friends is bidirectional, so if one person accepts a request from another person, both of them will have one more friend.

Thus, we can union column requester_id and accepter_id, and then count the number of the occurrence of each person.

select requester_id as ids from request_accepted
union all
select accepter_id from request_accepted;


Note: Here we should use union all instead of union because union all will keep all the records even the 'duplicated' one.

Taking the sample as an example, the output is:

ids
1
1
2
3
2
3
3
4

Then it will be fairly easy to get the 'ids' with most occurrence using the same technique as mentioned in problem 580. Customer Placing the Largest Number of Orders.

MySQL

select ids as id, cnt as num
from
(
select ids, count(*) as cnt
from
(
select requester_id as ids from request_accepted
union all
select accepter_id from request_accepted
) as tbl1
group by ids
) as tbl2
order by cnt desc
limit 1
;