  In this paper, we prove the boundedness of all the solutions for the equation
$\ddot{x}+n^2x+g(x)+\psi'(x)=p(t)$ with the Lazer-Leach condition on $g$ and
$p$, where $n\in \mathbb{N^+}$, $p(t)$ and $\psi'(x)$ are periodic and $g(x)$
is bounded. For the critical situation that $\big |\int_0^{2\pi}p(t)e^{int}dt
\big|=2\big|g(+\infty)-g(-\infty)\big|$, we also prove a sufficient and
necessary condition for the boundedness if $\psi'(x)\equiv0$.
