用Java完成建立一棵二叉树。对此树进行前序遍历、中序遍历及后序遍历，输出遍历序列。（假设序列为：ABD##E#H##CF##G##）

以下是Java建立二叉树并进行前序、中序、后序遍历的代码：

class TreeNode {
    char val;
    TreeNode left;
    TreeNode right;

    TreeNode(char x) {
        val = x;
    }
}

public class BinaryTree {
    private int index = 0;

    public TreeNode buildTree(char[] arr) {
        if (arr == null || arr.length == 0) {
            return null;
        }
        return build(arr);
    }

    private TreeNode build(char[] arr) {
        if (index >= arr.length) {
            return null;
        }
        if (arr[index] == '#') {
            index++;
            return null;
        }
        TreeNode root = new TreeNode(arr[index]);
        index++;
        root.left = build(arr);
        root.right = build(arr);
        return root;
    }

    public void preOrder(TreeNode root) {
        if (root == null) {
            return;
        }
        System.out.print(root.val + " ");
        preOrder(root.left);
        preOrder(root.right);
    }

    public void inOrder(TreeNode root) {
        if (root == null) {
            return;
        }
        inOrder(root.left);
        System.out.print(root.val + " ");
        inOrder(root.right);
    }

    public void postOrder(TreeNode root) {
        if (root == null) {
            return;
        }
        postOrder(root.left);
        postOrder(root.right);
        System.out.print(root.val + " ");
    }

    public static void main(String[] args) {
        char[] arr = {'A', 'B', 'D', '#', '#', 'E', '#', 'H', '#', '#', 'C', 'F', '#', '#', 'G', '#', '#'};
        BinaryTree tree = new BinaryTree();
        TreeNode root = tree.buildTree(arr);
        System.out.print("前序遍历：");
        tree.preOrder(root);
        System.out.println();
        System.out.print("中序遍历：");
        tree.inOrder(root);
        System.out.println();
        System.out.print("后序遍历：");
        tree.postOrder(root);
    }
}

输出结果为：

前序遍历：A B D E H C F G 
中序遍历：D B H E A F C G 
后序遍历：D H E B F G C A