如果您正在编写白板代码、进行面试，或者只是计划使用树，那么这些内容就有点冗长了。

应该进一步说，树不像Pair那样存在的原因是，你应该将你的数据封装在使用它的类中，最简单的实现是这样的:

/***
/* Within the class that's using a binary tree for any reason. You could 
/* generalize with generics IFF the parent class needs different value types.
 */
private class Node {
  public String value;
  public Node[] nodes; // Or an Iterable<Node> nodes;
}

这就是任意宽度的树。

如果你想要一个二叉树，它通常更容易使用命名字段:

private class Node { // Using package visibility is an option
  String value;
  Node left;
  Node right;
}

或者如果你想要一个trie

private class Node {
  String value;
  Map<char, Node> nodes;
}

现在你说你想要

给定一个表示给定节点的输入字符串，能够获得所有的子节点(某种类型的列表或字符串数组)

听起来像是你的家庭作业。 但既然我有理由相信任何最后期限都已经过去了……

import java.util.Arrays;
import java.util.ArrayList;
import java.util.List;

public class kidsOfMatchTheseDays {
 static private class Node {
   String value;
   Node[] nodes;
 }

 // Pre-order; you didn't specify.
 static public List<String> list(Node node, String find) {
   return list(node, find, new ArrayList<String>(), false);
 }

 static private ArrayList<String> list(
     Node node,
     String find,
     ArrayList<String> list,
     boolean add) {
   if (node == null) {
     return list;
   }
   if (node.value.equals(find)) {
     add = true;
   }
   if (add) {
     list.add(node.value);
   }
   if (node.nodes != null) {
     for (Node child: node.nodes) {
       list(child, find, list, add);
     }
   }
   return list;
 }

 public static final void main(String... args) {
   // Usually never have to do setup like this, so excuse the style
   // And it could be cleaner by adding a constructor like:
   //     Node(String val, Node... children) {
   //         value = val;
   //         nodes = children;
   //     }
   Node tree = new Node();
   tree.value = "root";
   Node[] n = {new Node(), new Node()};
   tree.nodes = n;
   tree.nodes[0].value = "leftish";
   tree.nodes[1].value = "rightish-leafy";
   Node[] nn = {new Node()};
   tree.nodes[0].nodes = nn;
   tree.nodes[0].nodes[0].value = "off-leftish-leaf";
   // Enough setup
   System.out.println(Arrays.toString(list(tree, args[0]).toArray()));
 }
}

这让你使用:

$ java kidsOfMatchTheseDays leftish
[leftish, off-leftish-leaf]
$ java kidsOfMatchTheseDays root
[root, leftish, off-leftish-leaf, rightish-leafy]
$ java kidsOfMatchTheseDays rightish-leafy
[rightish-leafy]
$ java kidsOfMatchTheseDays a
[]

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;
  }
}

import java.util.Collection;
import java.util.LinkedList;
import java.util.function.BiConsumer;
import java.util.function.Function;

/**
 * @author changjin wei(魏昌进)
 * @since 2021/7/15
 */
public class TreeUtils {

    private TreeUtils() {
    }

    /**
     * @param collection this is a collection of elements
     * @param getId this is a getId Function
     * @param getParentId this is a getParentId Function
     * @param setNode this is a setNode BiConsumer
     * @param <E> the type of elements in this collection
     * @param <R> the type of the result of the function
     *
     * @return Collection
     */
    public static <E, R> Collection<E> tree(Collection<E> collection, Function<E, R> getId, Function<E, R> getParentId, BiConsumer<E, Collection<E>> setNode) {
        Collection<E> root = new LinkedList<>();
        for (E node : collection) {
            R parentId = getParentId.apply(node);
            R id = getId.apply(node);
            Collection<E> elements = new LinkedList<>();
            boolean isParent = true;
            for (E element : collection) {
                if (id.equals(getParentId.apply(element))) {
                    elements.add(element);
                }
                if (isParent && getId.apply(element).equals(parentId)) {
                    isParent = false;
                }
            }
            if (isParent) {
                root.add(node);
            }
            setNode.accept(node, elements);
        }
        return root;
    }
}

我编写了一个树库，它可以很好地使用Java8，并且没有其他依赖项。它还提供了对函数式编程的一些思想的松散解释，并允许您映射/过滤/修剪/搜索整个树或子树。

https://github.com/RutledgePaulV/prune

这个实现在索引方面没有做任何特别的事情，而且我也没有偏离递归，所以使用大型树的性能可能会下降，可能会破坏堆栈。但如果你所需要的只是一个简单的小到中等深度的树，我认为它已经足够好了。它提供了一个健全的(基于值的)相等定义，它还有一个toString实现，可以让您可视化树!

你可以使用Java的任何XML API作为文档和节点..因为XML是一个带有字符串的树结构

    // TestTree.java
// A simple test to see how we can build a tree and populate it
//
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.tree.*;

public class TestTree extends JFrame {

  JTree tree;
  DefaultTreeModel treeModel;

  public TestTree( ) {
    super("Tree Test Example");
    setSize(400, 300);
    setDefaultCloseOperation(EXIT_ON_CLOSE);
  }

  public void init( ) {
    // Build up a bunch of TreeNodes. We use DefaultMutableTreeNode because the
    // DefaultTreeModel can use it to build a complete tree.
    DefaultMutableTreeNode root = new DefaultMutableTreeNode("Root");
    DefaultMutableTreeNode subroot = new DefaultMutableTreeNode("SubRoot");
    DefaultMutableTreeNode leaf1 = new DefaultMutableTreeNode("Leaf 1");
    DefaultMutableTreeNode leaf2 = new DefaultMutableTreeNode("Leaf 2");

    // Build our tree model starting at the root node, and then make a JTree out
    // of it.
    treeModel = new DefaultTreeModel(root);
    tree = new JTree(treeModel);

    // Build the tree up from the nodes we created.
    treeModel.insertNodeInto(subroot, root, 0);
    // Or, more succinctly:
    subroot.add(leaf1);
    root.add(leaf2);

    // Display it.
    getContentPane( ).add(tree, BorderLayout.CENTER);
  }

  public static void main(String args[]) {
    TestTree tt = new TestTree( );
    tt.init( );
    tt.setVisible(true);
  }
}