二叉樹的數(shù)據(jù)結(jié)構(gòu)就是一個個結(jié)點，節(jié)點內(nèi)部有指向左右子結(jié)點。

@Data
public class Node {
    private Object data;
    private Node leftChild;
    private Node rightChild;
    public Node(Object data) {
        this.data = data;
    }
    public Node(Object data, Node leftChild, Node rightChild) {
        this.data = data;
        this.leftChild = leftChild;
        this.rightChild = rightChild;
    }
    @Override
    public String toString() {
        return "Node{" +
                "data=" + data +
                ", leftChild=" + leftChild +
                ", rightChild=" + rightChild +
                '}';
    }
}

二叉樹通過遞歸實現(xiàn)先序，中序和后序排列

public class BinaryTreeImpl implements BinaryTree {
    private Node root;
    public BinaryTreeImpl(Node root) {
        this.root = root;
    }
    @Override
    public boolean isEmpty() {
        return null == root;
    }
    @Override
    public int size() {
        return size(root);
    }
    private int size(Node node) {
        if (null != node) {
            int ls = size(node.getLeftChild());
            int rs = size(node.getRightChild());
            return ls + rs + 1;
        } else {
            return 0;
        }
    }
    @Override
    public int getHeight() {
        return getHeight(root);
    }
    private int getHeight(Node node) {
        if (null != node) {
            int ls = getHeight(node.getLeftChild());
            int rs = getHeight(node.getRightChild());
            return ls > rs ? ls + 1 : rs + 1;
        } else {
            return 0;
        }
    }
    @Override
    public Node findKey(Node node) {
        return findKey(node, root);
    }
    private Node findKey(Node node, Node root) {
        if (null != root) {
            if (root.getData() == node.getData()) {
                return root;
            }
            Node result = findKey(node, root.getLeftChild());
            if (null != result) {
                return result;
            }
            result = findKey(node, root.getRightChild());
            if (null != result) {
                return result;
            }
        }
        return null;
    }
    @Override
    public void preOrderTraverse() {
        System.out.println("Start preOrderTraverse");
        preOrderTraverse(root);
    }
    private void preOrderTraverse(Node node) {
        if (node != null) {
            System.out.print(node.getData() + " ");
            preOrderTraverse(node.getLeftChild());
            preOrderTraverse(node.getRightChild());
        }
    }
    @Override
    public void postOrderTraverse() {
        System.out.println("Start postOrderTraverse");
        postOrderTraverse(root);
    }
    private void postOrderTraverse(Node node) {
        if (node != null) {
            postOrderTraverse(node.getLeftChild());
            postOrderTraverse(node.getRightChild());
            System.out.print(node.getData() + " ");
        }
    }
    @Override
    public void innerOrderTraverse() {
        System.out.println("Start innerOrderTraverse");
        innerOrderTraverse(root);
    }
    private void innerOrderTraverse(Node node) {
        if (node != null) {
            innerOrderTraverse(node.getLeftChild());
            System.out.print(node.getData() + " ");
            innerOrderTraverse(node.getRightChild());
        }
    }
}