diff --git a/include/s0009_palindrome_number.hpp b/include/s0009_palindrome_number.hpp
index 4d11182..6d12351 100644
--- a/include/s0009_palindrome_number.hpp
+++ b/include/s0009_palindrome_number.hpp
@@ -2,6 +2,7 @@
 #define S0009_PALINDROME_NUMBER
 
 #include <string>
+#include <algorithm>
 
 using namespace std;