Official solution
In \[\Delta\]OAC
\[\sin \left( {{\pi \over 4}} \right) = {1 \over {AC}}\]
\[ \Rightarrow AC = \sqrt 2 \]
Also, \[\tan {\pi \over 4} = {{OA} \over {OC}} = {1 \over {OC}}\]
\[\Rightarrow\] OC = 1
\[\therefore\] centre (0, 1); Radius = \[\sqrt 2 \]