  We consider the problem of sparse phase retrieval, where a $k$-sparse signal
${\bf x} \in {\mathbb R}^n \textrm{ (or } {\mathbb C}^n\textrm{)}$ is measured
as ${\bf y} = |{\bf Ax}|,$ where ${\bf A} \in {\mathbb R}^{m \times n} \textrm{
(or } {\mathbb C}^{m \times n}\textrm{ respectively)}$ is a measurement matrix
and $|\cdot|$ is the element-wise absolute value. For a real signal and a real
measurement matrix ${\bf A}$, we show that $m = 2k$ measurements are necessary
and sufficient to recover ${\bf x}$ uniquely. For complex signal ${\bf x} \in
{\mathbb C}^n$ and ${\bf A} \in {\mathbb C}^{m \times n}$, we show that $m =
4k-2$ phaseless measurements are sufficient to recover ${\bf x}$. It is known
that the multiplying constant $4$ in $m = 4k-2$ cannot be improved.
