Simplify the expression $$\frac{s^{2} + t^{2} + 2st - u^{2}}{s^{2} - t^{2} - 2tu - u^{2}}$$, provided $$\left(s + t + u\right) \neq 0$$.

A

$$\frac{s + t - u}{s - t -u}$$

B

$$\frac{s + t + u}{s - t +u}$$

C

$$\frac{s - t - u}{s + t -u}$$

D

$$\frac{s - t + u}{s + t +u}$$