/**

  * Definition for a binary tree node.

  * struct TreeNode {

  *     int val;

  *     TreeNode *left;

  *     TreeNode *right;

  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}

  * };

  */

 class Solution {

 public:

     TreeNode* invertTree(TreeNode* root) {

         if(!root)   return ;//重要在这而已

         if(root->left)    invertTree(root->left);

         if(root->right)   invertTree(root->right);

         TreeNode* tmp=root->right;

         root->right=root->left;

         root->left=tmp;

         return root;

     }

 };

 # Definition for a binary tree node.

 # class TreeNode(object):

 #     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 root!=None:

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

         return root

 # Definition for a binary tree node.

 # class TreeNode(object):

 #     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

         """

         stack=[root]

         while stack!=[]:

             back=stack.pop()

             if back!=None:

                 back.left, back.right= back.right, back.left

                 stack.extend([back.left,back.right])#也可以写成stack+=back.left,back.right

         return root