diff --git a/data/progress.txt b/data/progress.txt
index 2d4e4c64..1de65b0d 100644
--- a/data/progress.txt
+++ b/data/progress.txt
@@ -1,6 +1,7 @@
+20240229: 39
 20240228: 20,21,23,33(again)
 20240227: 1,3,5,4,19
-20231218: 57(todo),58(todo),268,61(todo),297
+20231218: 57(todo),58(todo),268,61(stodo),297
 20231211: 39,53,48,56
 20231208: 33
 20231206: 19,003
diff --git a/data/to_review.txt b/data/to_review.txt
index c20ed727..0579ba1e 100644
--- a/data/to_review.txt
+++ b/data/to_review.txt
@@ -1,41 +1,47 @@
+2024-04-24 -> ['39']
 2024-04-23 -> ['20,21,23,33(again)']
 2024-04-22 -> ['1,3,5,4,19']
+2024-04-03 -> ['39']
 2024-04-02 -> ['20,21,23,33(again)']
 2024-04-01 -> ['1,3,5,4,19']
+2024-03-21 -> ['39']
 2024-03-20 -> ['20,21,23,33(again)']
 2024-03-19 -> ['1,3,5,4,19']
+2024-03-13 -> ['39']
 2024-03-12 -> ['20,21,23,33(again)']
 2024-03-11 -> ['1,3,5,4,19']
+2024-03-08 -> ['39']
 2024-03-07 -> ['20,21,23,33(again)']
 2024-03-06 -> ['1,3,5,4,19']
+2024-03-05 -> ['39']
 2024-03-04 -> ['20,21,23,33(again)']
-2024-03-03 -> ['1,3,5,4,19']
-2024-03-02 -> ['20,21,23,33(again)']
-2024-03-01 -> ['20,21,23,33(again)', '1,3,5,4,19']
+2024-03-03 -> ['39', '1,3,5,4,19']
+2024-03-02 -> ['39', '20,21,23,33(again)']
+2024-03-01 -> ['39', '20,21,23,33(again)', '1,3,5,4,19']
 2024-02-29 -> ['20,21,23,33(again)', '1,3,5,4,19']
 2024-02-28 -> ['1,3,5,4,19']
-2024-02-11 -> ['57(todo),58(todo),268,61(todo),297']
+2024-02-11 -> ['57(todo),58(todo),268,61(stodo),297']
 2024-02-04 -> ['39,53,48,56']
 2024-01-30 -> ['19,003']
 2024-01-29 -> ['23']
 2024-01-28 -> ['55,45']
-2024-01-21 -> ['57(todo),58(todo),268,61(todo),297']
+2024-01-21 -> ['57(todo),58(todo),268,61(stodo),297']
 2024-01-14 -> ['39,53,48,56']
 2024-01-09 -> ['19,003']
-2024-01-08 -> ['57(todo),58(todo),268,61(todo),297', '23']
+2024-01-08 -> ['57(todo),58(todo),268,61(stodo),297', '23']
 2024-01-07 -> ['55,45']
 2024-01-02 -> ['452,406,135']
 2024-01-01 -> ['39,53,48,56']
-2023-12-31 -> ['57(todo),58(todo),268,61(todo),297', '134']
+2023-12-31 -> ['57(todo),58(todo),268,61(stodo),297', '134']
 2023-12-28 -> ['53']
 2023-12-27 -> ['19,003']
-2023-12-26 -> ['57(todo),58(todo),268,61(todo),297', '23']
+2023-12-26 -> ['57(todo),58(todo),268,61(stodo),297', '23']
 2023-12-25 -> ['55,45']
 2023-12-24 -> ['39,53,48,56']
-2023-12-23 -> ['57(todo),58(todo),268,61(todo),297']
-2023-12-21 -> ['57(todo),58(todo),268,61(todo),297']
-2023-12-20 -> ['57(todo),58(todo),268,61(todo),297']
-2023-12-19 -> ['57(todo),58(todo),268,61(todo),297', '39,53,48,56', '19,003']
+2023-12-23 -> ['57(todo),58(todo),268,61(stodo),297']
+2023-12-21 -> ['57(todo),58(todo),268,61(stodo),297']
+2023-12-20 -> ['57(todo),58(todo),268,61(stodo),297']
+2023-12-19 -> ['57(todo),58(todo),268,61(stodo),297', '39,53,48,56', '19,003']
 2023-12-18 -> ['23']
 2023-12-17 -> ['55,45']
 2023-12-16 -> ['39,53,48,56']
diff --git a/doc/cheatsheet/backtrack.md b/doc/cheatsheet/backtrack.md
index 1d0e6f5f..e2592547 100644
--- a/doc/cheatsheet/backtrack.md
+++ b/doc/cheatsheet/backtrack.md
@@ -299,7 +299,12 @@
             else{ 
                 for(int i = start; i < nums.length; i++){
                     tempList.add(nums[i]);
-                    backtrack(list, tempList, nums, remain - nums[i], i); // not i + 1 because we can reuse same elements
+                    /** NOTE !!!
+                     *
+                     *   use i, since we need to use start from current (i) index in recursion call
+                     *    (reuse current index)
+                     */
+                    backtrack(list, tempList, nums, remain - nums[i], i);
                     tempList.remove(tempList.size() - 1);
                 }
             }
diff --git a/leetcode_java/src/main/java/LeetCodeJava/BackTrack/CombinationSum.java b/leetcode_java/src/main/java/LeetCodeJava/BackTrack/CombinationSum.java
index ebaf5aa5..0b0f8b90 100644
--- a/leetcode_java/src/main/java/LeetCodeJava/BackTrack/CombinationSum.java
+++ b/leetcode_java/src/main/java/LeetCodeJava/BackTrack/CombinationSum.java
@@ -18,6 +18,7 @@ public List<List<Integer>> combinationSum(int[] candidates, int target) {
             return null;
         }
 
+        // NOTE !!! we sore here
         Arrays.sort(candidates);
 
         List<List<Integer>> res = new ArrayList<>();
@@ -46,7 +47,11 @@ private void backTrack(int[] candidates, int target, List<Integer> tmp, List<Lis
         for (int i = idx; i < candidates.length; i++){
             int cur = candidates[i];
             tmp.add(cur);
-            /** NOTE !!! need to use start from i index in recursion call */
+            /** NOTE !!!
+             *
+             *    use i, since we need to use start from current (i) index in recursion call
+             *    (reuse current index)
+             */
             backTrack(candidates, target, tmp, res, i);
             // undo
             tmp.remove(tmp.size()-1);