Compute build order from dependencies

You are given a directed graph represented as a mapping where each key X maps to a list of tasks that depend on X (i.e., X must be completed before each listed task). Example: A: [B, C], B: [D, E], C: [D]. Implement solve(A) that outputs a valid execution order of all tasks reachable from A so that each task appears after all of its prerequisites. If multiple valid orders exist, break ties alphabetically. Detect and report cycles, and state the time and space complexity of your approach.