c# - Best way to get an ordered list of groups by value from an unordered list -

i'd know if there's more efficient way ordered list of groups value unordered list, using groupby() followed orderby(), this:

list<int> list = new list<int>(); ienumerable<ienumerable<int>> orderedgroups = list.groupby(x => x).orderby(x => x.key); 

for more detail, have large list<t> i'd sort, there lots of duplicate values want return results ienumerable<ienumerable<t>>, groupby() returns ienumerable of groups. if use orderby(), ienumerable<t>, no easy way know whether value has changed 1 item next. group list sort groups, list large ends being slow. since orderby() returns orderedenumerable can sorted on secondary field using thenby(), must internally distinguish between adjacent items same or different values.

is there way can make use of fact orderedenumerable<t> must internally group results value (in order facilitate thenby()), or otherwise what's efficient way use linq ordered list of groups?

• you can use tolookup, returns ienumerable<igrouping<tkey, telement> , orderby values of each key on demand. o(n) create lookup , o(h) order elements under each group (values key) assuming h number of elements under group

• you can improve performance amortized o(n) using idictionary<tkey, iorderedenumerable<t>>. if want order multiple properties, again o(h) on group. see this answer more info on iorderedenumerable. can use sortedlist<tkey, tvalue> instead of iorderedenumerable

[update]:

here another answer can take look. again, involves doing orderby on top of result.

further, can come own data structure don't see data structure available on bcl meeting requrement.

one possible implementation:

you can have binary search tree search/delete/insert in o(longn) on average. , doing in-order traversal give sorted keys. each node on tree have ordered collection example, values.

node looks this:

public class mynode {     prop string key;     prop sortedcollection mycollection; } 

you can traverse on initial collection once , create special data structure can queried fast results.

[update 2]: if have possible keys below 100k, feel implementing own data structure overkill. order return pretty fast , time taken tiny. unless have large data , order multiple times, tolookup should work well.