"Write an expression for the probability that in a sequence of r random integers in the range [1,n] no integer occurs more than once"

Is my answer right?

Let the sequence be represented by $\displaystyle a_{n}= a_{1}, a_{2}, ... a_{r}$ where $\displaystyle r<=n$

$\displaystyle P(a_{1}=a_{2}) = \frac{1}{n}$

$\displaystyle P(a_{1}!=a_{2}) = 1-\frac{1}{n}$

So $\displaystyle P(a_{1}!=a_{2}!=...!=a_{r}) = (1-\frac{1}{n})^r$