一、前序遍历

　　访问顺序：先根节点，再左子树，最后右子树；上图的访问结果为：GDAFEMHZ。

1）递归实现

public void preOrderTraverse1(TreeNode root) {
	if (root != null) {
      System.out.print(root.val + "->");
      preOrderTraverse1(root.left);
      preOrderTraverse1(root.right);
   }
}

2）非递归实现

public void preOrderTraverse2(TreeNode root) {
	Stack<TreeNode> stack = new Stack<>();
    TreeNode node = root;
    while (node != null || !stack.empty()) {
        if (node != null) {
            System.out.print(node.val + "->");
            stack.push(node);
            node = node.left;
        } else {
            TreeNode tem = stack.pop();
            node = tem.right;
        }
    }
}

二、中序遍历

　　访问顺序：先左子树，再根节点，最后右子树；上图的访问结果为：ADEFGHMZ。

1）递归实现

public void inOrderTraverse(TreeNode root) {
    if (root != null) {
        inOrderTraverse(root.left);
        System.out.print(root.val + "->");
        inOrderTraverse(root.right);
    }
}

2）非递归实现

public void inOrderTraverse(TreeNode root) {
    Stack<TreeNode> stack = new Stack<>();
    TreeNode node = root;
    while (node != null || !stack.isEmpty()) {
        if (node != null) {
            stack.push(node);
            node = node.left;
        } else {
            TreeNode tem = stack.pop();
            System.out.print(tem.val + "->");
            node = tem.right;
        }
    }
}

三、后序遍历

　　访问顺序：先左子树，再右子树，最后根节点，上图的访问结果为：AEFDHZMG。

1）递归实现

public void postOrderTraverse(TreeNode root) {
    if (root != null) {
        postOrderTraverse(root.left);
        postOrderTraverse(root.right);
        System.out.print(root.val + "->");
    }
}

2）非递归实现

public void postOrderTraverse(TreeNode root) {
	TreeNode cur, pre = null;

    Stack<TreeNode> stack = new Stack<>();
    stack.push(root);

    while (!stack.empty()) {
        cur = stack.peek();
        if ((cur.left == null && cur.right == null) || (pre != null && (pre == cur.left || pre == cur.right))) {
            System.out.print(cur.val + "->");
            stack.pop();
            pre = cur;
        } else {
            if (cur.right != null)
                stack.push(cur.right);
            if (cur.left != null)
                stack.push(cur.left);
        }
    }
}

四、层次遍历

　　访问结果：GDMAFHZE。

public void levelOrderTraverse(TreeNode root) {
    if (root == null) {
        return;
    }
    Queue<TreeNode> queue = new LinkedList<TreeNode>();
    queue.add(root);

    while (!queue.isEmpty()) {
        TreeNode node = queue.poll();
        System.out.print(node.val + "->");

        if (node.left != null) {
            queue.add(node.left);
        }
        if (node.right != null) {
            queue.add(node.right);
        }
    }
}

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