Trees and Wall

A few trees are planted randomly at different cells of a square grid. A masson is told to build walls around all the trees such that - Adjacent walls are perpendicular - Total area bounded by the walls is minimum A 2D matrix of dimension $NXN$ is used to represent the grid with values $0$, $1$ or $2$ where - $0$ indicates an empty cells - $1$ indicates location of a tree - $2$ indicates part of