$g$ is a primitive root of $p^s$, then all the solutions of the congruence $x^{p-1}\equiv 1 \pmod p^{s}$ are given by $1, g^{p^{s}},\ldots, g^{p^{s}(s-2)}$
Clearly that the given set of solutions fit the congruence, but how do I show any solution of the congruence is in the form as listed?
Use Discrete Logarithm,
$(p-1)$ind$_gx\equiv0\pmod{\phi(p^s)}$
$\iff(p-1)$ind$_gx\equiv0\pmod{p^{s-1}(p-1)}$
$\iff$ind$_gx\equiv0\pmod{p^{s-1}}$
$\iff x\equiv g^{kp^{s-1}}\pmod{p^s}$