Cho hàm số \[f\left( x \right) = \left\{ \begin{array}{l} - x\,\,\,\,\,\,\,n\^e 'u\,\,\,\,x < 0\\\sqrt x \,\,\,\,\,\,n\^e 'u\,\,\,\,x \ge 0.\end{array} \right.\] Tính \(\mathop {\lim }\limits_{x \to {0^ + }} f\left( x \right)\), \(\mathop {\lim }\limits_{x \to {0^ - }} f\left( x \right)\) và \(\mathop {\lim }\limits_{x \to 0} f\left( x \right)\).