Question 129
It is given the set $S = \{1, 2, 3, \dots, 3n\}$, where $n$ is a positive integer. $T$ is a subset of $S$ such that: for any $x, y, z \in T$ (where $x, y, z$ can be the same), $x+y+z \notin T$. Find the maximum value of the number of elements in such set.