A circle touches both the y-axis and the line \( x + y = 0 \). Then the locus of its center is:
\( y = \sqrt{2} \, x \)
\( x = \sqrt{2} \, y \)
\( y^{2} - x^{2} = 2xy \)
\( x^{2} - y^{2} = 2xy \)