Problem $3$: concatenate$($lst$1,$ lst$2)$

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 $1->2->3->7->8->9.$