Indian Statistical Institute B.Math & B.Stat : Polynomials

If a polynomial $P$ with integer coefficients has three distinct integer zeroes, then show that $P(n) \neq 1$ for any integer. Let $\alpha, \beta$ and $\gamma$ be the distinct integer zeroes of the polynomial $P$. If possible let $P(m)=1$ where $m \in \mathbb{Z}$. Since $P \in \mathbb{Z}(x)$ we have $\alpha - m | P(\alpha)-P(m) \implies \alpha - m | (-1)$. Similarly $\beta - m | (-1)$ and $\gamma - m | (-1)$. Since $\alpha, \beta$ and $\gamma$ are distinct $\alpha-m, \beta-m$ and $\gamma-m$ are distinct. This shows that $\alpha-m, \beta-m$ and $\gamma-m$ are distinct factors of $1$, which is impossible! So the assumption that $P(m)=1$ is not tenable for any integer $m$.