$f(n)$ is a function counting all the ones that show up in the sequence $1, 2, 3, ..., n$.
IE $f(1)=1$, $f(10)=2$, $f(11)=4$ etc. Discounting the trivial case $f(1) = 1$, when is the first time $f(n)=n$?
I found this on an interview question. I was wondering if there were any clever ways to think about this mathematically/cleverly other than brute-forcing the problem.