The value of $\lim _ { n \rightarrow \infty } \frac { [ r ] + [ 2 r ] + \ldots + [ n r ] } { n ^ { 2 } }$, where $r$ is non-zero real number and $[ r ]$ denotes the greatest integer less than or equal to $r$, is equal to:\\
(1) $\frac { r } { 2 }$\\
(2) $r$\\
(3) $2 r$\\
(4) 0