java无限级树生成算法，空间复杂度为O(2n) java算法

生成树的一般算法为遍历集合，获取父类，再依次递归遍历集合获取其子类，这次的升级算法大致如此，但是在遍历之前先进行了一次分组，将所有父级id相同的分为一组，遍历父级时直接根据id从map中获取其子级，这样一来，就之遍历了n次，算上分组遍历的n次，时间复杂度仅为O(2n)；
树工厂类：

/** * 树工厂类，用于生成树 * @author xieshuang * @date 2019-08-08 10:43 */ public class TreeFactory{int a; /** * 优化方案 * 1.按照pid进行分组 * @param treeNodes * @return */public Collection createTree(Collection treeNodes){ a = 0; long l2 = System.currentTimeMillis(); Map> collect = treeNodes.stream().collect(Collectors.groupingBy(T::getPid)); System.out.println("第一次遍历花费："+(System.currentTimeMillis()-l2)); long l3 = System.currentTimeMillis(); Collection treeNodeList = new ArrayList<>(); List list; List list1 = collect.get(0); List list2 = collect.get(0L); if (list1==null){ list = list2; }else { list = list1; } if (list!=null){ for (T t : list) { treeNodeList.add(addChildNode(t, collect)); } } System.out.println("第二次遍历花费："+(System.currentTimeMillis()-l3)); System.out.println("总共遍历了："+a); return treeNodeList; }private T addChildNode(T treeNode, Map> collect){ a++; List list = collect.get(treeNode.getId()); if (list!=null){ List treeNodeList = new ArrayList<>(); for (T t : list) { treeNodeList.add(addChildNode(t, collect)); } treeNode.setChilds(treeNodeList); } return treeNode; }}

树父类：

@Data public class Tree{ private Number id; private Number pid; private Collection childs; }

自定义的树类：

@Data public class MyTree extends Tree { private String name; }

public class TreeFactoryTest {public static void main(String[] args) { List trees = new ArrayList<>(); for (int i = 1; i < 100; i++) { MyTree myTree = new MyTree(); myTree.setName("顶级"+i); myTree.setId(i); myTree.setPid(0); trees.add(myTree); for (int j = 0; j <100 ; j++) { MyTree myTree1 = new MyTree(); myTree1.setName("子级"+j); myTree1.setId(j+(i*100)); myTree1.setPid(i); trees.add(myTree1); for (int k = 0; k <100 ; k++) { MyTree myTree2 = new MyTree(); myTree2.setName("子级"+j); myTree2.setId(k+(j*100)+(i*10000)); myTree2.setPid(j+(i*100)); trees.add(myTree2); } } } System.out.println(trees.size()); Collection tree = new TreeFactory().createTree(trees); //System.out.println(JSONObject.toJSONString(tree)); } }

【java无限级树生成算法，空间复杂度为O(2n)】测试结果：
百万数据量耗时0.2秒