Graph the piecewise-defined function to determine whether it is a one-to-one function. If it is a one-to-one function, find its inverse.
$$f(x)=\left\{\begin{array}{ll}
-x & x<-2 \\
\sqrt{4-x^{2}} & -2 \leq x \leq 0 \\
-\frac{1}{x} & x>0
\end{array}\right.$$