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Each sort algorithm will be the job, but it’s a OVERKILL.
For input like:
aa
cc
aa
bb
dd
bb
cc
I just need something like:
aa
aa
cc
cc
bb
bb
dd
The order of each pattern is not required.
Is there such an algorithm for this kind of job?
You simply want to use a hashtable here, or more abstractly an associative array. Iterate over the input, adding it to the hashtable with value (tag, if you prefer) of 1 if it hasn’t yet been seen, or incrementing the count by one if it already exists in the hashtable.
The algorithm is thus O(n) in both time and space, which is as good as you could reasonably expect. I recommend some reading up on hashtables, as it is a highly useful data structure that appears in all sorts of places in algorithm and software design.