int countLeafNodes(TreeNode* root) {
    if (root == nullptr) return 0;

    stack<TreeNode*> s;
    s.push(root);
    int count = 0;
    while (!s.empty()) {
        TreeNode* node = s.top();
        s.pop();

        if (node->left == nullptr && node->right == nullptr) {
            count++;
        }

        if (node->right) s.push(node->right);
        ________________ // 在此处填入代码
    }
    return count;
}