Invert a binary tree.

Example:

Input:

4
   /   \
  2     7
 / \   / \
1   3 6   9

Output:

4
   /   \
  7     2
 / \   / \
9   6 3   1

Trivia:
This problem was inspired by ​​​this original tweet​​​ by ​​Max Howell​​:

题解：

class Solution {
public:
   TreeNode* invertTree(TreeNode* root) {
      if (root == NULL) {
            return NULL;
        }
      else {
          TreeNode *index = root->left;
          root->left = invertTree(root->right);
         root->right = invertTree(index);
            return root;
        }
   }
};