递归

• 思路：
  • 从根开始递归遍历二叉树，叶节点开始翻转；
  • 如果遍历到的当前的 root 节点左右两棵子树已经翻转，交换左右子树即可；

/*** Definition for a binary tree node.* struct TreeNode {*     int val;*     TreeNode *left;*     TreeNode *right;*     TreeNode() : val(0), left(nullptr), right(nullptr) {}*     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}*     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}* };*/
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;}
};