I have a question if I have my $k=300$and my loop is like this :for( int x = 0 ; x<n ; x--){    for(int y=0 ; y<k; y++){        ...    }}Is this still $O(n^2)$? If no, why?Thank you :)
Big O time complexity
time complexity;landau notation
Assuming that you meant x++ rather than x--, the complexity is $\Theta(nk)$. Since $k = 300$, you can simplify that to $\Theta(n)$. As Ran G. mentions, since $n = O(n^2)$, then your loop is also $O(n^2)$. It's also $O(2^n)$. But the tight analysis is $\Theta(n)$.