Invert Binary Tree

可以divide conquer，也可以top-bottom

class Solution(object):
    def invertTree(self, root):
        """
        :type root: TreeNode
        :rtype: TreeNode
        """
        if not root:
            return None

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

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