Realizing degree imbalances in directed graphs

In a directed graph, the imbalance of a vertex v is b(v)=d+(v)−d−(v). We characterize the integer lists that can occur as the sequence of imbalances of a simple directed graph. For the realizable sequences, we determine the maximum number of arcs in a realization and provide a greedy algorithm that constructs realizations with the minimum number of arcs.