Function $4$: concatenate$($llst$1,$ llst$2)$

Given two $($possibly empty$)$ linked lists, concatenate them such that the first list comes first.

Example: When the first input linked list lst$1$ is $1->2->3$ and the second lst$2$ is $7->8->9,$ the output linked list

is $1->2->3->7->8$

std::list concatenate$($const std::list& list$1,$ const std::list& list$2)\{$

$//$ TODO: Student to implement

return std::list$()$; $//$ Placeholder return

$\}->9.$