递归获取某个节点的儿子节点

java 代码：

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public List<Department> getAllChildrenDepartmentsFlat(Long parentId) {         List<Department> allDepartments = departmentRepository.findAll();  // 假设使用JPA的Repository来进行数据库操作           List<Department> allChildren = new ArrayList<>();         Department parent = departmentRepository.findById(parentId);         if (parent != null) {             allChildren.add(parent);             flattenChildren(allDepartments, parent, allChildren);         }           return allChildren;     }       private void flattenChildren(List<Department> allDepartments, Department parent, List<Department> flatList) {         List<Department> children = allDepartments.stream()                 .filter(department -> department.getParentId().equals(parent.getId()))                 .collect(Collectors.toList());                   for (Department child : children) {             flatList.add(child);             flattenChildren(allDepartments, child, flatList);         }     }