高频题（0）
1. Build Dependency Task Scheduler
Design a task scheduler with the following requirements:
1. The input is a set of build dependencies in the format child: [parent1, parent2], which is the opposite of the format used in LeetCode's 'Course Schedule II'.
2. Given a target, and considering there are millions of rules, it is not necessary to build a map for all rules. Instead, create a smaller map based on the target using depth-first search (DFS).
3. Implement another map to determine the current executable tasks, similar to the in-degree map in 'Course Schedule II'.
4. For a given node, determine which tasks have been unblocked. This requires checking the children of the node because multiple tasks might be runnable in the entire map. Specifically, identify which children have an in-degree of zero.
