solution.md

# 翻转二叉树

<p>翻转一棵二叉树。</p>

<p><strong>示例：</strong></p>

<p>输入：</p>

<pre>     4
   /   \
  2     7
 / \   / \
1   3 6   9</pre>

<p>输出：</p>

<pre>     4
   /   \
  7     2
 / \   / \
9   6 3   1</pre>

<p><strong>备注:</strong><br>
这个问题是受到 <a href="https://twitter.com/mxcl" target="_blank">Max Howell </a>的 <a href="https://twitter.com/mxcl/status/608682016205344768" target="_blank">原问题</a> 启发的 ：</p>

<blockquote>谷歌：我们90％的工程师使用您编写的软件(Homebrew)，但是您却无法在面试时在白板上写出翻转二叉树这道题，这太糟糕了。</blockquote>


## template

```python
class TreeNode:
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None


class Solution(object):
    def invertTree(self, root):
        """
		:type root: TreeNode
		:rtype: TreeNode
		"""

        if not root:
            return None
        root.left, root.right = root.right, root.left

        self.invertTree(root.left)
        self.invertTree(root.right)

        return root
```

## 答案

```python
class TreeNode:
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None


class Solution(object):
    def invertTree(self, root):
        """
		:type root: TreeNode
		:rtype: TreeNode
		"""

        if not root:
            return None
        root.left, root.right = root.right, root.left

        self.invertTree(root.left)
        self.invertTree(root.right)

        return root
```

## 选项

### A

```python
class TreeNode:
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None


class Solution(object):
    def invertTree(self, root):
        """
		:type root: TreeNode
		:rtype: TreeNode
		"""

        if not root:
            return None
        self.invertTree(root.left)
        self.invertTree(root.right)
        root.left, root.right = root.right, root.left

        return root
```

### B

```python
class TreeNode:
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None


class Solution(object):
    def invertTree(self, root):
        """
		:type root: TreeNode
		:rtype: TreeNode
		"""

        if not root:
            return None
        self.invertTree(root.left)
        root.left, root.right = root.right, root.left
        self.invertTree(root.right)

        return root
```

### C

```python
class TreeNode:
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None


class Solution(object):
    def invertTree(self, root):
        """
		:type root: TreeNode
		:rtype: TreeNode
		"""

        if not root:
            return None
        self.invertTree(root.right)
        root.left, root.right = root.right, root.left
        self.invertTree(root.left)

        return root
```