Produce the Build Order

medium

Same monorepo, harder ask: the CI system now needs the actual build order, not just a yes/no.

There are n packages numbered 0 to n - 1, and each rule [a, b] in deps means b must be built before a. Return a list of all n package numbers in a valid build order. If the rules are contradictory (cyclic), return an empty list [].

The test inputs are constructed so the valid order is unique — return exactly that order.

Example 1

in n = 3, deps = [[1, 0], [2, 1]]

out [0, 1, 2]

A chain: 0 unlocks 1, which unlocks 2.

Example 2

in n = 2, deps = [[0, 1], [1, 0]]

out []

Mutual dependency — no order exists.

Constraints

· 1 <= n <= 100
· 0 <= len(deps) <= 500
· deps[i] = [a, b] with 0 <= a, b < n
· when an order exists, it is unique