运用一个递归进行二叉树的翻转，很基本的题目。、

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