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) {
            invertTree(root->left);
            invertTree(root->right);

            TreeNode *copy = root->left;
            root->left = root->right;
            root->right = copy;
        }

        return root;
    }
};