我对所有这些方法都有意见。

我使用的是“MappedTreeStructure”实现。这个实现很好地重新组织了树，并且不包含节点的“副本”。

但是没有提供分级方法。

看看那些有问题的输出!

MutableTree<String> tree = new MappedTreeStructure<>();

        tree.add("0", "1");
        tree.add("0", "2");
        tree.add("0", "3");
        tree.add("0", "4");
        tree.add("0", "5");

        tree.add("2", "3");
        tree.add("2", "5");

        tree.add("1", "2");
        tree.add("1", "3");
        tree.add("1", "5");

        System.out.println(
                tree.toString()
        );

哪个输出:(错误)

-  0
  -  1
    -  2
    -  3
    -  5
  -  4

还有这个:(正确)

tree = new MappedTreeStructure<>();

        tree.add("0", "1");
        tree.add("0", "2");
        tree.add("0", "3");
        tree.add("0", "4");
        tree.add("0", "5");

        tree.add("1", "2");
        tree.add("1", "3");
        tree.add("1", "5");

        tree.add("2", "3");
        tree.add("2", "5");

        System.out.println(
                tree.toString()
        );

正确的输出:

-  0
  -  1
    -  2
      -  3
      -  5
  -  4

如此!我创建了另一个实现来欣赏。请给一些建议和反馈!

package util;

import java.util.HashMap;
import java.util.Map;

public class Node<N extends Comparable<N>> {

    public final Map<N, Node<N>> parents = new HashMap<>();
    public final N value;
    public final Map<N, Node<N>> children = new HashMap<>();

    public Node(N value) {
        this.value = value;
    }
}

package util;

import java.util.*;
import java.util.stream.Collectors;

public class HierarchyTree<N extends Comparable<N>> {

    protected final Map<N, Node<N>> nodeList = new HashMap<>();

    public static <T extends Comparable<T>> Node<T> state(Map<T, Node<T>> nodeList, T node) {
        Node<T> tmp = nodeList.getOrDefault(node, new Node<>(node));
        nodeList.putIfAbsent(node, tmp);
        return tmp;
    }

    public static <T extends Comparable<T>> Node<T> state(Map<T, Node<T>> nodeList, Node<T> node) {
        Node<T> tmp = nodeList.getOrDefault(node.value, node);
        nodeList.putIfAbsent(node.value, tmp);
        return tmp;
    }

    public Node<N> state(N child) {
        return state(nodeList, child);
    }

    public Node<N> stateChild(N parent, N child) {
        Node<N> pai = state(parent);
        Node<N> filho = state(child);
        state(pai.children, filho);
        state(filho.parents, pai);
        return filho;
    }

    public List<Node<N>> addChildren(List<N> children) {
        List<Node<N>> retorno = new LinkedList<>();
        for (N child : children) {
            retorno.add(state(child));
        }
        return retorno;
    }

    public List<Node<N>> addChildren(N parent, List<N> children) {
        List<Node<N>> retorno = new LinkedList<>();
        for (N child : children) {
            retorno.add(stateChild(parent, child));
        }
        return retorno;
    }

    public List<Node<N>> addChildren(N parent, N... children) {
        return addChildren(parent, Arrays.asList(children));
    }

    public List<Node<N>> getRoots() {
        return nodeList.values().stream().filter(value -> value.parents.size() == 0).collect(Collectors.toList());
    }

    @Override
    public String toString() {
        return deepPrint("- ");
    }

    public String deepPrint(String prefix) {
        StringBuilder builder = new StringBuilder();
        deepPrint(builder, prefix, "", getRoots());
        return builder.toString();
    }

    protected void deepPrint(StringBuilder builder, String prefix, String sep, List<Node<N>> node) {
        for (Node<N> item : node) {
            builder.append(sep).append(item.value).append("\n");
            deepPrint(builder, prefix, sep + prefix, new ArrayList<>(item.children.values()));
        }
    }

    public SortedMap<Long, Set<N>> tree() {
        SortedMap<Long, Set<N>> tree = new TreeMap<>();
        tree(0L, tree, getRoots());
        return tree;
    }

    protected void tree(Long i, SortedMap<Long, Set<N>> tree, List<Node<N>> roots) {
        for (Node<N> node : roots) {
            Set<N> tmp = tree.getOrDefault(i, new HashSet<>());
            tree.putIfAbsent(i, tmp);
            tmp.add(node.value);
            tree(i + 1L, tree, new ArrayList<>(node.children.values()));
        }
    }

    public void prune() {
        Set<N> nodes = new HashSet<>();
        SortedMap<Long, Set<N>> tree = tree();
        List<Long> treeInverse = tree.keySet().stream().sorted(Comparator.reverseOrder()).collect(Collectors.toList());
        for (Long treeItem : treeInverse) {
            for (N n : tree.get(treeItem)) {
                Map<N, Node<N>> children = nodeList.get(n).children;
                for (N node : nodes) {
                    children.remove(node);
                }
                nodes.addAll(children.keySet());
            }
        }
    }

    public static void main(String[] args) {
        HierarchyTree<Integer> tree = new HierarchyTree<>();
        tree.addChildren(Arrays.asList(1, 2, 3, 4, 5));
        tree.addChildren(1, Arrays.asList(2, 3, 5));
        tree.addChildren(2, Arrays.asList(3, 5));
        tree.prune();
        System.out.println(tree);

        tree = new HierarchyTree<>();
        tree.addChildren(Arrays.asList(1, 2, 3, 4, 5));
        tree.addChildren(2, Arrays.asList(3, 5));
        tree.addChildren(1, Arrays.asList(2, 3, 5));
        tree.prune();
        System.out.println(tree);
    }
}

输出总是正确的:

1
- 2
- - 3
- - 5
4

1
- 2
- - 3
- - 5
4

我写了一个处理通用树的小库。它比秋千轻多了。我也有一个专门的项目。

这个呢?

import java.util.ArrayList;
import java.util.Collection;
import java.util.HashMap;

/**
  * @author ycoppel@google.com (Yohann Coppel)
  * 
  * @param <T>
  *          Object's type in the tree.
*/
public class Tree<T> {

  private T head;

  private ArrayList<Tree<T>> leafs = new ArrayList<Tree<T>>();

  private Tree<T> parent = null;

  private HashMap<T, Tree<T>> locate = new HashMap<T, Tree<T>>();

  public Tree(T head) {
    this.head = head;
    locate.put(head, this);
  }

  public void addLeaf(T root, T leaf) {
    if (locate.containsKey(root)) {
      locate.get(root).addLeaf(leaf);
    } else {
      addLeaf(root).addLeaf(leaf);
    }
  }

  public Tree<T> addLeaf(T leaf) {
    Tree<T> t = new Tree<T>(leaf);
    leafs.add(t);
    t.parent = this;
    t.locate = this.locate;
    locate.put(leaf, t);
    return t;
  }

  public Tree<T> setAsParent(T parentRoot) {
    Tree<T> t = new Tree<T>(parentRoot);
    t.leafs.add(this);
    this.parent = t;
    t.locate = this.locate;
    t.locate.put(head, this);
    t.locate.put(parentRoot, t);
    return t;
  }

  public T getHead() {
    return head;
  }

  public Tree<T> getTree(T element) {
    return locate.get(element);
  }

  public Tree<T> getParent() {
    return parent;
  }

  public Collection<T> getSuccessors(T root) {
    Collection<T> successors = new ArrayList<T>();
    Tree<T> tree = getTree(root);
    if (null != tree) {
      for (Tree<T> leaf : tree.leafs) {
        successors.add(leaf.head);
      }
    }
    return successors;
  }

  public Collection<Tree<T>> getSubTrees() {
    return leafs;
  }

  public static <T> Collection<T> getSuccessors(T of, Collection<Tree<T>> in) {
    for (Tree<T> tree : in) {
      if (tree.locate.containsKey(of)) {
        return tree.getSuccessors(of);
      }
    }
    return new ArrayList<T>();
  }

  @Override
  public String toString() {
    return printTree(0);
  }

  private static final int indent = 2;

  private String printTree(int increment) {
    String s = "";
    String inc = "";
    for (int i = 0; i < increment; ++i) {
      inc = inc + " ";
    }
    s = inc + head;
    for (Tree<T> child : leafs) {
      s += "\n" + child.printTree(increment + indent);
    }
    return s;
  }
}

在这里:

public class Tree<T> {
    private Node<T> root;

    public Tree(T rootData) {
        root = new Node<T>();
        root.data = rootData;
        root.children = new ArrayList<Node<T>>();
    }

    public static class Node<T> {
        private T data;
        private Node<T> parent;
        private List<Node<T>> children;
    }
}

这是一个基本的树结构，可用于String或任何其他对象。实现简单的树来满足您的需要是相当容易的。

您需要添加的只是用于添加、删除、遍历和构造函数的方法。节点是树的基本构建块。

首先应该定义什么是树(对于域)，最好先定义接口。并不是所有的树结构都是可修改的，能够添加和删除节点应该是一个可选的功能，所以我们为此做了一个额外的接口。

没有必要创建保存值的节点对象，事实上，我认为这是大多数树实现中的主要设计缺陷和开销。如果查看Swing, TreeModel没有节点类(只有DefaultTreeModel使用TreeNode)，因为实际上并不需要它们。

public interface Tree <N extends Serializable> extends Serializable {
    List<N> getRoots ();
    N getParent (N node);
    List<N> getChildren (N node);
}

可变树结构(允许添加和删除节点):

public interface MutableTree <N extends Serializable> extends Tree<N> {
    boolean add (N parent, N node);
    boolean remove (N node, boolean cascade);
}

有了这些接口，使用树的代码就不必太关心树是如何实现的。这允许您使用通用实现和专用实现，在专用实现中，通过将函数委托给另一个API来实现树。

例如:文件树结构

public class FileTree implements Tree<File> {

    @Override
    public List<File> getRoots() {
        return Arrays.stream(File.listRoots()).collect(Collectors.toList());
    }

    @Override
    public File getParent(File node) {
        return node.getParentFile();
    }

    @Override
    public List<File> getChildren(File node) {
        if (node.isDirectory()) {
            File[] children = node.listFiles();
            if (children != null) {
                return Arrays.stream(children).collect(Collectors.toList());
            }
        }
        return Collections.emptyList();
    }
}

示例:通用树结构(基于父/子关系):

public class MappedTreeStructure<N extends Serializable> implements MutableTree<N> {

    public static void main(String[] args) {

        MutableTree<String> tree = new MappedTreeStructure<>();
        tree.add("A", "B");
        tree.add("A", "C");
        tree.add("C", "D");
        tree.add("E", "A");
        System.out.println(tree);
    }

    private final Map<N, N> nodeParent = new HashMap<>();
    private final LinkedHashSet<N> nodeList = new LinkedHashSet<>();

    private void checkNotNull(N node, String parameterName) {
        if (node == null)
            throw new IllegalArgumentException(parameterName + " must not be null");
    }

    @Override
    public boolean add(N parent, N node) {
        checkNotNull(parent, "parent");
        checkNotNull(node, "node");

        // check for cycles
        N current = parent;
        do {
            if (node.equals(current)) {
                throw new IllegalArgumentException(" node must not be the same or an ancestor of the parent");
            }
        } while ((current = getParent(current)) != null);

        boolean added = nodeList.add(node);
        nodeList.add(parent);
        nodeParent.put(node, parent);
        return added;
    }

    @Override
    public boolean remove(N node, boolean cascade) {
        checkNotNull(node, "node");

        if (!nodeList.contains(node)) {
            return false;
        }
        if (cascade) {
            for (N child : getChildren(node)) {
                remove(child, true);
            }
        } else {
            for (N child : getChildren(node)) {
                nodeParent.remove(child);
            }
        }
        nodeList.remove(node);
        return true;
    }

    @Override
    public List<N> getRoots() {
        return getChildren(null);
    }

    @Override
    public N getParent(N node) {
        checkNotNull(node, "node");
        return nodeParent.get(node);
    }

    @Override
    public List<N> getChildren(N node) {
        List<N> children = new LinkedList<>();
        for (N n : nodeList) {
            N parent = nodeParent.get(n);
            if (node == null && parent == null) {
                children.add(n);
            } else if (node != null && parent != null && parent.equals(node)) {
                children.add(n);
            }
        }
        return children;
    }

    @Override
    public String toString() {
        StringBuilder builder = new StringBuilder();
        dumpNodeStructure(builder, null, "- ");
        return builder.toString();
    }

    private void dumpNodeStructure(StringBuilder builder, N node, String prefix) {
        if (node != null) {
            builder.append(prefix);
            builder.append(node.toString());
            builder.append('\n');
            prefix = "  " + prefix;
        }
        for (N child : getChildren(node)) {
            dumpNodeStructure(builder, child, prefix);
        }
    }
}

Java中没有适合您需求的特定数据结构。您的需求非常具体，因此需要设计自己的数据结构。看看你的需求，任何人都可以说你需要某种具有特定功能的n元树。你可以通过以下方式设计你的数据结构:

Structure of the node of the tree would be like content in the node and list of children like: class Node { String value; List children;} You need to retrieve the children of a given string, so you can have 2 methods 1: Node searchNode(String str), will return the node that has the same value as given input (use BFS for searching) 2: List getChildren(String str): this method will internally call the searchNode to get the node having same string and then it will create the list of all string values of children and return. You will also be required to insert a string in tree. You will have to write one method say void insert(String parent, String value): this will again search the node having value equal to parent and then you can create a Node with given value and add to the list of children to the found parent.

我建议，你写一个类的节点结构类节点{字符串值;在另一个NodeUtils类中列出children;}和所有其他方法，如search, insert和getChildren，这样你也可以传递树的根来对特定的树执行操作，例如: 类NodeUtils{公共静态节点搜索(节点根，字符串值){//执行BFS并返回节点}