11. Given $S_{1}=1, S_{2}=1-2, S_{3}=1-2+3, S_{4}=1-2+3-4, S_{5}=1-2+3-4+5, \cdots$, then $S_{1}+S_{2}+S_{3}+\cdots+S_{299}=$ $\qquad$.
【Answer】150
【Analysis】Sequence calculation + parity of positive and negative numbers (or rounding)
$$
S_{1}=1, \quad S_{2}=-1, \quad S_{3}=2, \quad S_{4}=-2, \quad S_{5}=3, \quad S_{6}=-3, \cdots \cdots
$$

The pattern is quite obvious. We can see that $S_{1}+S_{2}+S_{3}+S_{4}+\cdots+S_{298}=0$, and $S_{n}=\left[\frac{n}{2}\right]$ (rounding up), so, $S_{299}=(299+1) \div 2=150$.