输入：
     4
   /   \
  2     7
 / \   / \
1   3 6   9
输出：
     4
   /   \
  7     2
 / \   / \
9   6 3   1

 - 递归到每个节点
 - 将每个节点的左右节点调换

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     struct TreeNode *left;
 *     struct TreeNode *right;
 * };
 */
// - 递归到每个节点
// - 将每个节点的左右节点调换
struct TreeNode* invertTree(struct TreeNode* root) {
    if (root == NULL) {
        return NULL;
    }
    struct TreeNode* left = invertTree(root->left);
    struct TreeNode* right = invertTree(root->right);
    root->left = right;
    root->right = left;
    return root;
}