For integers $m\geq 3$, we study the non-self-adjoint eigenvalue problems
$-u^{\prime\prime}(x)+(x^m+P(x))u(x)=E u(x)$, $0\leq x<+\infty$, with the
boundary conditions $u(+\infty)=0$ and $\alpha u(0)+\beta u^{\prime}(0)=0$ for
some $\alpha, \beta\in\C$ with $|\alpha|+|\beta|\not=0$, where $P(x)=a_1
x^{m-1}+a_2 x^{m-2}+...+a_{m-1} x$ is a polynomial. We provide asymptotic
expansions of the eigenvalue counting function and the eigenvalues $E_{n}$.
Then we apply these to the inverse spectral problem, reconstructing some
coefficients of polynomial potentials from asymptotic expansions of the
eigenvalues.