list转tree

/**
 * list tree node
 * @Author: zilong
 * @Date: 2021/11/25
 */
@Data
public class Node<T> {
    private T id;
    private T parentId;
}
/**
 * tree node
 * @Author: zilong
 * @Date: 2021/11/24
 */
@Data
public class TreeNode<T, R extends TreeNode> extends Node<T> {
    private List<R> children;
}
/**
 * list to tree
 * @param list list
 * @param <R> 具体类型
 * @return
 */
public static <R extends TreeNode> List<R> list2Tree(List<R> list) {
    Map<Object, R> nodeMap = new HashMap<>();
    List<R> headNodes = new ArrayList<>();
    List<R> tree = new ArrayList<>();
    // 缓存节点
    list.forEach(e -> {
        nodeMap.put(e.getId(), e);
        // 根节点缓存
        if (e.getParentId() == null) {
            headNodes.add(e);
        }
    });
    // 节点连接到各个根节点上
    list.forEach(e -> {
        if (e.getParentId() != null) {
            if (nodeMap.get(e.getParentId()).getChildren() == null) {
                nodeMap.get(e.getParentId()).setChildren(new ArrayList());
            }
            nodeMap.get(e.getParentId()).getChildren().add(e);
        }
    });
    // 根节点返回
    headNodes.forEach(node -> {
        tree.add(nodeMap.get(node.getId()));
    });
    return tree;
}