Linked Lists -some tips ?

I need some tip, how to create 3 linked lists and first list should contain only the lowercase characters of the read string, the second only the uppercase characters of the read string.
Thanks...

Use three std::list<char>, and then read by using std::string objects. What is the third linked list for?

The third list should contain all other characters of the read string and then i have to sort the created lists by using the merge sort algorithm... And i find it difficult because i'm begginer. haha