diff --git a/test009.cpp b/test009.cpp
index 81cbdcf..99634a5 100644
--- a/test009.cpp
+++ b/test009.cpp
@@ -18,7 +18,7 @@ public:
 
         if(checkCnt(cnts, cntp)) { ret.push_back(0);}
 
-        for(int r = p.length(); r < s.length(); r++){
+        for(int r = p.length(); r < s.length(); r++){ // sliding window
             int l = r - p.length() + 1;
             cnts[s[l-1] - 'a']--;
             cnts[s[r] - 'a']++;