Exercise $1$:

Suppose that an intermixed sequence of $($stack$)$ push and pop operations are performed. The pushes push the integers $0$ through $9$ in order; the pops print out the return value. Which of the following sequence$($s$)$ could not occur?

$($e$)1234569870$

$($f$)0465381729$

$($g$)1479865302$

$($h$)2143658790$