翻转一棵二叉树。

示例：

输入：

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

输出：

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

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

此方法用到的是递归调用，注意定义一个二叉树的方式,此方法有些慢

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