Build Order

You are given a list of projects and a list of dependencies (pairs where the second project depends on the first). All dependencies must be built before the dependent project.

Find a build order that satisfies all dependencies. If no valid order exists (cycle), return None.

Example:

projects: ["a","b","c","d","e","f"]
dependencies: [("a","d"),("f","b"),("b","d"),("f","a"),("d","c")]
Output: ["f","e","b","a","d","c"]  (one valid order)