Data structures in programming language with linear types
Linearity is not a sufficient constraint to pin down a unique stateful representation, and so the answer to your question depends on how you interpret linear logic in terms of state. This will typically be reflected in how you must interpret the $!A$ modality. 

If your intended semantics of references says that all pointers are unique values (i.e., there is at most a single reference to an object) then dags and graph structures are not expressible, for the sort of tautological reason that a dag may contain multiple references to the same object. In this case, $!A$ must be a computation which creates a new value of type $A$, since you want maps $\delta_A : !A \multimap !A \otimes !A$ and $\epsilon_A : !A \multimap A$.

However, suppose that you want $!A$ to represent sharing. Then, objects can be garbage-collected with reference counting, with the maps  $\delta_A : !A \multimap !A \otimes !A$ and $\epsilon_A : !A \multimap A$ can be realized as operations which just bump reference counts. In this case, you can't use linearity to assume that it's always safe to mutate values, since there's sharing. But you can ensure that all memory allocation is explicit in your program, and that there are no cycles in the heap. 

Most practical implementations of linear types use neither of these two interpretations. Instead, references are viewed as freely duplicable entities, and what we track linearly are in fact capabilities. Capabilities are not runtime values; they are purely conceptual entities which are intended to represent the permission to access a reference. The idea is that you program in a permission-passing style, and so even if there are many references to the same object, a read or modification of a piece of state can only occur if you also have the capability to access it. And since the capability is linear, you know that only you can change it. 

$$
\array{
\mathsf{new} &amp; : &amp; \forall \alpha. \;\alpha \multimap \exists c : \iota. \mathsf{cap}(c) \otimes \mathsf{ref}(\alpha, c) \\
\mathsf{get} &amp; : &amp; \forall \alpha, c:\iota. \;\mathsf{cap}(c) \otimes \mathsf{ref}(\alpha, c) \multimap \alpha \otimes \mathsf{cap}(c) \otimes \mathsf{ref}(\alpha, c) \\
\mathsf{set} &amp; : &amp; \forall \alpha, c:\iota. \;\mathsf{cap}(c) \otimes \mathsf{ref}(\alpha, c) \otimes \alpha \multimap \mathsf{cap}(c) \otimes \mathsf{ref}(\alpha, c) \\
\mathsf{copy} &amp; : &amp; \forall \alpha, c:\iota.\; \mathsf{ref}(\alpha,c) \multimap \mathsf{ref}(\alpha,c) \otimes \mathsf{ref}(\alpha,c)
}
$$

In the API sketched above, $c$ ranges over $\iota$, some domain of compile-time indices, and $\alpha$ ranges over types. We have a type $\mathsf{cap}(c)$ which is a capability indexed by $c$, and a type $\mathsf{ref}(\alpha, c)$, which is a type of references to $\alpha$ accessed by a capability $c$. Calling $\mathsf{get}$ and $\mathsf{set}$ on a reference requires the capability $c$, and calling $\mathsf{new}$ creates a new reference and a new capability sharing a common index. However, $\mathsf{copy}$-ing a reference does not require access to any capability, so anyone can copy a reference as long as they don't look inside it.