diff --git a/data_structures/avltree.cpp b/data_structures/avltree.cpp
index 4efd1207c..251f3606c 100644
--- a/data_structures/avltree.cpp
+++ b/data_structures/avltree.cpp
@@ -6,9 +6,9 @@
  * \warning This program is a poor implementation and does not utilize any of
  * the C++ STL features.
  */
-#include <algorithm>
-#include <iostream>
-#include <queue>
+#include <algorithm>  // for std::max
+#include <iostream>   // for std::cout
+#include <queue>      // for std::queue
 
 using node = struct node {
     int data;