数组转树形结构只需两步-松哥资源

示例标题

已很经典的菜单为例，假设结构如下：

id
parent_id
name
xxx1
xxx2
….

按照一比一构造一个类（只增加一个children字段）：

@Data
public class XXXX{
	private Long id;

	private Long parentId;

	private String name;

	private String xxx1;

	private String xxx2;

	private List<XXXX> children;
}

public List<XXXX> treeMenu() {
    List<XXXX> xxxxs = xxxxDao.menuList();

    //第一步分组
    Map<Long, List<XXXX>> parentId2Children =
        xxxxs.stream().collect(Collectors.groupingBy(i -> i.getParentId()));

    //第二步构造树
    List<XXXX> roots = parentId2Children.remove(0L);
    buildTree(roots, parentId2Children);
    
    return roots;
}

private void buildTree(List<XXXX> parents,
    Map<Long, List<XXXX>> parentId2Children) {
    if (CollUtil.isEmpty(parents)) {
        return;
    }

    parents.sort(Comparator.comparing(XXXX::getId));

    for (XXXX parent : parents) {
        List<XXXX> children = parentId2Children.remove(parent.getId());
        parent.setChildren(children);
        buildTree(children, parentId2Children);
    }
}