Means the algorithm is log linear. Quicksort is an example of a log linear algorithm.
If you write a function that is O(n*n) it is probably going to be quite slow with a large input size. A really quick function would be O(1)?. This would be something that only ever executes the same amount of code, no matter the input size.
BigONotation is hardly the only way to describe the complexity of an algorithm, there is also little oh notation, theta notation and others. However, they are rarely, if ever, used. BigONotation is the standard way to describe how a function scales to its input size.
Also, see An informal introduction to O(N) notation.
Someone might want to merge this and BigOhNotation?.
3 pages link to BigONotation:
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