How to find the time complexity of interpolation and addition?

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I have signal p(t) with N samples, where I need to advance by t0 seconds. There are two ways to perform,

  1. By converting into discete system like p(n+round(t0/dt)), where dt is sample time and round(t0/dt) is number of samples to be advanced.

  2. Second approach is to directly apply interpolation like interp in matlab. p(t+t0) = interp1(t,p(t), t+t0, 'spline') where t+t0 is query points.

I am reading several literature and found for interpolation complexity of O(N* log(N)), where N is total number of samples overwhich interpolation has to be performed. But I dont know why log(N) exists?

Second, in the approach 1, is the complexity just O(N)?