二叉树java实现

/**
 * 链表实现二叉树
 * -创建二叉树
 * -前序、中序、后序、层次遍历二叉树
 * -判断值是否存在
 * -二叉树高度
 */
public class BinaryTree {
    // 链表节点
    public static class Node {
        public String val;
        public Node left;
        public Node right;
        public Node() {
        }
        public Node(String val) {
            this.val = val;
        }
    }

    /**
     * 创建二叉树（需要辅助变量 i）
     * 通过前序字符串 String[] {"A", "B","#", "#", "C", "#", "#"} 来创建
     * 注意：必须是前序字符串
     * #表示空节点
     */
    private static int i = 0;

    // 利用前序字符串数组自动创建
    public static Node createBTree(String[] arr) {
        if ("#".equals(arr[i])) {
            i++;
            return null;
        }
        Node root = new Node(arr[i++]);
        root.left = createBTree(arr);
        root.right = createBTree(arr);
        return root;
    }

    // 手动创建(输入顺序必须是前序序列)
    public static Node handCreate() {
        Scanner scanner = new Scanner(System.in);
        System.out.println("请输入节点元素：");
        String next = scanner.next();
        Node root = new Node(next);
        if ("#".equals(next)) return null;
        root.left = handCreate();
        root.right = handCreate();
        return root;
    }

    // 前序遍历
    public static void preOrder(Node root) {
        if (root == null) return;
        System.out.print(root.val + "\t");
        preOrder(root.left);
        preOrder(root.right);
    }

    // 中序遍历
    public static void inOrder(Node root) {
        if (root == null) return;
        inOrder(root.left);
        System.out.print(root.val + "\t");
        inOrder(root.right);
    }

    // 后序遍历
    public static void afterOrder(Node root) {
        if (root == null) return;
        afterOrder(root.left);
        afterOrder(root.right);
        System.out.print(root.val + "\t");
    }

    // 层次遍历
    public static void levelOrder(Node root) {
        Queue<Node> queue = new LinkedList<>();
        // 先将根节点入队
        queue.add(root);
        // 遍历，直到队列为空
        while (!queue.isEmpty()) {
            // 出队输出
            Node pollNode = queue.poll();
            System.out.print(pollNode.val + "\t");
            // 如果左子树不为空，入队
            if (pollNode.left != null) {
                queue.add(pollNode.left);
            }
            // 如果右子树不为空，入队
            if (pollNode.right != null) {
                queue.add(pollNode.right);
            }
        }
    }

    // 二叉树中是否存在指定值
    public static boolean isExist(Node root, String val) {
        if (root == null) return false;
        if (val.equals(root.val)) return true;
        boolean l = isExist(root.left, val);
        boolean r = isExist(root.right, val);
        return l || r;
    }

    // 二叉树高度
    public static int getHeight(Node root) {
        if (root == null) return 0;
        int lHeight = getHeight(root.left);
        int rHeight = getHeight(root.right);
        return Math.max(lHeight, rHeight) + 1;
    }

    // main
    public static void main(String[] args) {
        // 前序字符串数组
        String[] arr1 = {"A", "B", "D", "#", "#", "E", "#", "#", "C", "#", "#"};
        String[] arr2 = {"A", "B", "#", "#", "C", "#", "#"};
        Node root = BinaryTree.createBTree(arr2);
        System.out.println("前序遍历：");
        BinaryTree.preOrder(root);
        System.out.println();

        System.out.println("中序遍历：");
        BinaryTree.inOrder(root);
        System.out.println();

        System.out.println("后序遍历：");
        BinaryTree.afterOrder(root);
        System.out.println();

        System.out.println("层次遍历：");
        BinaryTree.levelOrder(root);
        System.out.println();

        System.out.println("二叉树高度：" + BinaryTree.getHeight(root));
        System.out.println("是否存在：" + BinaryTree.isExist(root, "C"));
    }
}