Find unique lines

Question

How can I find the unique lines and remove all duplicates from a file? My input file is

1
1
2
3
5
5
7
7

I would like the result to be:

Answer 1

While sort takes O(n log(n)) time, I prefer using

awk '!seen[$0]++'

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awk '!seen[$0]++' is an abbreviation for awk '!seen[$0]++ {print}', print line(=$0) if seen[$0] is not zero. It take more space but only O(n) time.