Use a set instead of a list, and a copy of it to count (i, j) and (j, i) once.
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added 2 commits
September 22, 2021 15:06
Lookups are O(n) in lists and O(1) in dictionaries and sets. I'll use a set because we don't need to associate values. To count "(i, j)" and "(j, i)" once, we'll look up "j" in a copy of the set "numbers" (instead of the "numbers" itself), and then we'll delete "i" from the copy. This way when "i:=j", we won't find "j:=i" in the copy. Note that we can't modify the set while we're iterating over it, and also the numbers have to reset for each time we call "func(x)" (these are other reasons I use a copy of "numbers"). This solution is merely convenient and neither memory nor time-efficient.
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This solution is merely for convenience and is neither memory nor time-efficient:
Lookups are O(n) in lists and O(1) in dictionaries and sets (because dictionaries and sets are implemented as hash tables). I used a set because we don't need to associate values (and also sets use less memory than dictionaries).
To count
(i, j)and(j, i)once, I look upjin a copy of the setnumbers(instead of thenumbersitself), and then deleteifrom the copy. This way wheni:=j, we won't findj:=iin the copy so it doesn't count. Also, we can't modify the set while we're iterating over it, andnumbershave to reset for each time we callfunc(x)(these are other reasons I used a copy).