Shortest-paths trees are not necessarily unique. Figure 2 gives two shortest-paths trees rooted
at vertex a for the graph from Figure 1. Take a look at the paths from a to e. In Figure 2(a), it
goes from a to g, and then g to e. In Figure 2(b), it goes from a to b, b to d, and then d to e. Both
of them are of length 7, which is the length of a shortest path from a to e. Notice that the total
edge weight of two shortest-paths trees may not be the same. For example, the total edge weight of
the shortest-paths tree in Figure 2(a) is 18, whereas that of the shortest-paths tree in Figure 2(b)
is 17.