In the merge-sort tree shown in Figures 11.2 through 11.4, some edges are drawn as arrows. What is the meaning of a downward arrow? How about an upward arrow?

Figure 11.2: Visualization of an execution of merge-sort. Each node of the tree represents a recursive call of merge-sort. The nodes drawn with dashed lines represent calls that have not been made yet. The node drawn with thick lines represents the current call. The empty nodes drawn with thin lines represent completed calls. The remaining nodes (drawn with thin lines and not empty) represent calls that are waiting for a child invocation to return. (Continues in Figure 11.3.)

Figure 11.3: Visualization of an execution of merge-sort. (Continues in Figure 11.4.)

Figure 11.4: Visualization of an execution of merge-sort. Several invocations are

omitted between (l) and (m) and between (m) and (n). Note the conquer step performed

in step (p).