代码实现：

方法1：先序遍历

/*** Definition for a binary tree node.* struct TreeNode {*     int val;*     struct TreeNode *left;*     struct TreeNode *right;* };*/// 交换左右子树
void swap(struct TreeNode *root) {struct TreeNode *l = root->left;struct TreeNode *r = root->right;root->left = r;root->right = l;
}// 递归：先序遍历
void pre_order(struct TreeNode *root) {if (root == NULL) {return ;}swap(root); // 交换pre_order(root->left);pre_order(root->right);
}struct TreeNode* invertTree(struct TreeNode *root) {pre_order(root);return root;
}

方法2：后序遍历

/*** Definition for a binary tree node.* struct TreeNode {*     int val;*     struct TreeNode *left;*     struct TreeNode *right;* };*/// 交换左右子树
void swap(struct TreeNode *root) {struct TreeNode *l = root->left;struct TreeNode *r = root->right;root->left = r;root->right = l;
}// 递归：后序遍历
void pre_order(struct TreeNode *root) {if (root == NULL) {return ;}pre_order(root->left);pre_order(root->right);swap(root); // 交换
}struct TreeNode* invertTree(struct TreeNode *root) {pre_order(root);return root;
}