\[ y(x) y''(x)-y'(x)^2-1=0 \] ✓ Mathematica : cpu = 0.175512 (sec), leaf count = 97
\[\left \{\left \{y(x)\to \frac {1}{2} e^{-e^{c_1} x-2 c_1-e^{c_1} c_2} \left (e^{2 e^{c_1} \left (c_2+x\right )}+e^{2 c_1}\right )\right \},\left \{y(x)\to \frac {1}{2} \left (e^{-e^{c_1} x-2 c_1-e^{c_1} c_2}+e^{e^{c_1} x+e^{c_1} c_2}\right )\right \}\right \}\]
✓ Maple : cpu = 0.279 (sec), leaf count = 79
\[ \left \{ y \left ( x \right ) ={\frac {{\it \_C1}}{2} \left ( {1 \left ( {{\rm e}^{{\frac {{\it \_C2}}{{\it \_C1}}}}} \right ) ^{-2} \left ( {{\rm e}^{{\frac {x}{{\it \_C1}}}}} \right ) ^{-2}}+1 \right ) {{\rm e}^{{\frac {{\it \_C2}}{{\it \_C1}}}}}{{\rm e}^{{\frac {x}{{\it \_C1}}}}}},y \left ( x \right ) ={\frac {{\it \_C1}}{2} \left ( \left ( {{\rm e}^{{\frac {{\it \_C2}}{{\it \_C1}}}}} \right ) ^{2} \left ( {{\rm e}^{{\frac {x}{{\it \_C1}}}}} \right ) ^{2}+1 \right ) \left ( {{\rm e}^{{\frac {{\it \_C2}}{{\it \_C1}}}}} \right ) ^{-1} \left ( {{\rm e}^{{\frac {x}{{\it \_C1}}}}} \right ) ^{-1}} \right \} \]