题目描述：

Invert a binary tree.

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

代码实现：

/**
 * 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==NULL){
            return NULL;
        }
        
        TreeNode* nl;
        TreeNode* nr;
        nl = invertTree(root->right);
        nr = invertTree(root->left);
        
        root->left = nl;
        root->right = nr;

        return root;
    }
};