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;
}