Java 高效构建树形结构

private List<SysDirectory> buildDirectoriesTree(List<SysDirectory> directories) {
    List<SysDirectory> returnList = new ArrayList<>();
    directories = directories.stream().sorted(Comparator.comparing(SysDirectory::getLevel).reversed()).collect(Collectors.toList());
    Map<String, List<SysDirectory>> directoryMap = new HashMap<>();
    for (SysDirectory directory : directories) {
        String parentId = String.valueOf(directory.getParentId());
        directoryMap.computeIfAbsent(parentId, k -> new ArrayList<>()).add(directory);
        Optional<List<SysDirectory>> children = Optional.ofNullable(directoryMap.get(directory.getDirectoryId()));
        children.ifPresent(directory::setChildren);
        if (directory.getLevel() == 1) {
            returnList.add(directory);
        }
    }
    return returnList;
}

@Data
public class SysDirectory extends BaseEntity {
    private static final long serialVersionUID = 1L;

    /**
     * 目标体系id
     */
    private String directoryId;

    /**
     * 父节点id
     */
    private Long parentId;

    /**
     * 祖级列表
     */
    private String ancestors;

    /**
     * 目标体系名称
     */
    private String directoryName;

    /**
     * 子集
     */
    private List<SysDirectory> children = new ArrayList<SysDirectory>();

    /**
     * 层级
     */
    private Integer level;

}