Quiz 10：極限存在的條件

\(f(x) = \left\{ {\begin{array}{*{20}{c}}{\;\;x\;,\;if\;x\;is\;rational}\\{ - x\;,if\;x\;is\;irrational}\end{array}} \right. \) 則極限 \(\lim\limits_{x \to {\rm{0}}} f(x)\) ？

(rational 有理數、irrational 無理數)