对于如上二叉树，分别使用前序遍历、中序遍历、后序遍历遍历出它的每一个节点
在此之前，先新建一个节点类，节点类属性包含当前节点的数据(字符)、当前节点的左、右节点。

当不对一个节点的左右节点赋值，由于他们是成员变量且为引用类型，有初始值null，左右子树不赋值即左右子树都为null即为叶子结点

1. 新建TreeNode节点类

public class TreeNode {
	private String Data;
	private TreeNode left;
	private TreeNode right;
	public String getData() {
		return Data;
	}
	public void setData(String data) {
		Data = data;
	}
	public TreeNode getLeft() {
		return left;
	}
	public void setLeft(TreeNode left) {
		this.left = left;
	}
	public TreeNode getRight() {
		return right;
	}
	public void setRight(TreeNode right) {
		this.right = right;
	}
	public TreeNode(String data, TreeNode left, TreeNode right) {
		super();
		Data = data;
		this.left = left;
		this.right = right;
	}
	public TreeNode() {
		super();
	}
	public TreeNode(String data) {
		super();
		Data = data;
	}
	@Override
	public String toString() {
		return Data;
	}
}

2. 遍历

对于前序遍历 先输出根节点 再输出左子树 最后输出右子树
对于输出子树可以用递归实现，当递归到节点为null递归结束

对于中序遍历 先输出左子树 再出根节点 最后输出右子树
对于输出子树可以用递归实现，当递归到节点为null递归结束

...
代码实现：

public class BinaryTreeErgodic {
	public static void beforePre(TreeNode cur) {
		if(cur==null) {
			return;
		}
		System.out.print(cur+" ");
		beforePre(cur.getLeft());
		beforePre(cur.getRight());
	}
	public static void afterPre(TreeNode cur) {
		if(cur==null) {
			return;
		}
		afterPre(cur.getLeft());
		afterPre(cur.getRight());
		System.out.print(cur+" ");
	}
	public static void midPre(TreeNode cur) {
		if(cur==null) {
			return;
		}
		midPre(cur.getLeft());
		System.out.print(cur+" ");
		midPre(cur.getRight());
	}
}

3.构建二叉树进行遍历

	public static void main(String[] args) {
		TreeNode a = new TreeNode("A");
		TreeNode b = new TreeNode("B");
		TreeNode c = new TreeNode("C");
		TreeNode d = new TreeNode("D");
		TreeNode e = new TreeNode("E");
		TreeNode f = new TreeNode("F");
		TreeNode g = new TreeNode("G");
		TreeNode h = new TreeNode("H");
		a.setLeft(b);a.setRight(c);
		b.setLeft(d);b.setRight(f);
		c.setLeft(g);c.setRight(h);
		d.setLeft(e);
		//BinaryTreeErgodic.afterPre(a);
		//BinaryTreeErgodic.midPre(a);
		//BinaryTreeErgodic.beforePre(a);
	}