Prove that the running time of an algorithm is $\Theta(g(n))$ if and only if its worst-case running time is $O(g(n))$ and its best-case running time is $\Omega(g(n))$.

If $T_w$ is the worst-case running time and $T_b$ is the best-case running time, we know that:

$$ 0 \leq c_1g(n) \leq T_b(n) \quad \text{for } n > n_b \\ 0 \leq T_w(n) \leq c_2g(n) \quad \text{for } n > n_w $$

Combining them we get:

$$ 0 \leq c_1g(n) \leq T_b(n) \leq T_w(n) \leq c_2g(n) \quad \text{for } n > \max(n_b, n_w) $$

Since the running time is bound between $T_b$ and $T_w$ and the above is the definition of the $\Theta$-notation, we have our proof.