public class Forest : MonoBehaviour
{
    public List<Tree> treeLists;

    public Tree getAvailableTree()
    {
        Tree availableTree = treeLists.Find((Tree t) => {
            return !t.isLumberjackAssign;
        });
        if(availableTree == null) {
            return null;
        }
        availableTree.isLumberjackAssign = true;
        return availableTree;
    }
}