diff --git a/src/main/java/g1801_1900/s1815_maximum_number_of_groups_getting_fresh_donuts/Solution.java b/src/main/java/g1801_1900/s1815_maximum_number_of_groups_getting_fresh_donuts/Solution.java
index 8060c27df..adf572e28 100644
--- a/src/main/java/g1801_1900/s1815_maximum_number_of_groups_getting_fresh_donuts/Solution.java
+++ b/src/main/java/g1801_1900/s1815_maximum_number_of_groups_getting_fresh_donuts/Solution.java
@@ -1,60 +1,89 @@
 package g1801_1900.s1815_maximum_number_of_groups_getting_fresh_donuts;
 
 // #Hard #Array #Dynamic_Programming #Bit_Manipulation #Bitmask #Memoization
-// #2022_05_03_Time_7_ms_(86.67%)_Space_43.6_MB_(73.33%)
+// #2025_02_21_Time_2_ms_(100.00%)_Space_41.56_MB_(100.00%)
 
-import java.util.Arrays;
 import java.util.HashMap;
 import java.util.Map;
 
 public class Solution {
-    private static final Map<String, Integer> MAP = new HashMap<>();
-
     public int maxHappyGroups(int batchSize, int[] groups) {
-        int[] count = new int[batchSize];
-        int res = 0;
-        int remGroup = 0;
-        for (int group : groups) {
-            int g = group % batchSize;
-            if (g == 0) {
-                res++;
-            } else if (count[batchSize - g] > 0) {
-                remGroup--;
-                res++;
-                count[batchSize - g]--;
-            } else {
-                count[g]++;
-                remGroup++;
-            }
+        if (batchSize == 1) {
+            return groups.length;
+        }
+        int[] withSize = new int[batchSize];
+        for (int size : groups) {
+            withSize[size % batchSize]++;
+        }
+        int fromZero = withSize[0];
+        withSize[0] = 0;
+        int fromEnds = 0;
+        for (int l = 1, r = batchSize - 1; l < r; l++, r--) {
+            int usable = Math.min(withSize[l], withSize[r]);
+            fromEnds += usable;
+            withSize[l] -= usable;
+            withSize[r] -= usable;
         }
-        res += dfs(0, remGroup, count, batchSize);
-        return res;
+        int fromMid = 0;
+        if (batchSize % 2 == 0) {
+            fromMid = withSize[batchSize / 2] / 2;
+            withSize[batchSize / 2] -= fromMid * 2;
+        }
+        return get(pruneEnd(withSize), batchSize, 0, new HashMap<>())
+                + fromZero
+                + fromEnds
+                + fromMid;
     }
 
-    private int dfs(int curr, int remain, int[] count, int batch) {
-        if (remain == 0) {
-            return 0;
+    private int get(int[] ar, int batchSize, int rem, Map<Long, Integer> cache) {
+        long hash = 0;
+        for (int e : ar) {
+            hash = hash * 69L + e;
         }
-        int res = 0;
-        String s = Arrays.toString(count);
-        if (MAP.containsKey(s)) {
-            return MAP.get(s);
+        Integer fromCache = cache.get(hash);
+        if (fromCache != null) {
+            return fromCache;
         }
-        if (curr == 0) {
-            res++;
-            curr = batch;
+        if (zeroed(ar)) {
+            cache.put(hash, 0);
+            return 0;
         }
-        int val = 0;
-        for (int i = 1; i < count.length; i++) {
-            if (count[i] == 0) {
+        int max = 0;
+        for (int i = 0; i < ar.length; i++) {
+            if (ar[i] == 0) {
                 continue;
             }
-            count[i]--;
-            val = Math.max(val, dfs((curr - i + batch) % batch, remain - 1, count, batch));
-            count[i]++;
+            ar[i]--;
+            int from = get(ar, batchSize, (rem + i) % batchSize, cache);
+            if (from > max) {
+                max = from;
+            }
+            ar[i]++;
+        }
+        int score = max + (rem == 0 ? 1 : 0);
+        cache.put(hash, score);
+        return score;
+    }
+
+    private int[] pruneEnd(int[] in) {
+        int endingZeros = 0;
+        for (int i = in.length - 1; i >= 0; i--) {
+            if (in[i] != 0) {
+                break;
+            }
+            endingZeros++;
+        }
+        int[] out = new int[in.length - endingZeros];
+        System.arraycopy(in, 0, out, 0, out.length);
+        return out;
+    }
+
+    private boolean zeroed(int[] ar) {
+        for (int e : ar) {
+            if (e != 0) {
+                return false;
+            }
         }
-        res += val;
-        MAP.put(s, res);
-        return res;
+        return true;
     }
 }
diff --git a/src/test/java/g1801_1900/s1815_maximum_number_of_groups_getting_fresh_donuts/SolutionTest.java b/src/test/java/g1801_1900/s1815_maximum_number_of_groups_getting_fresh_donuts/SolutionTest.java
index 2c140d800..7bd950983 100644
--- a/src/test/java/g1801_1900/s1815_maximum_number_of_groups_getting_fresh_donuts/SolutionTest.java
+++ b/src/test/java/g1801_1900/s1815_maximum_number_of_groups_getting_fresh_donuts/SolutionTest.java
@@ -16,4 +16,36 @@ void maxHappyGroups2() {
         assertThat(
                 new Solution().maxHappyGroups(4, new int[] {1, 3, 2, 5, 2, 2, 1, 6}), equalTo(4));
     }
+
+    @Test
+    void maxHappyGroups3() {
+        assertThat(
+                new Solution()
+                        .maxHappyGroups(
+                                7,
+                                new int[] {
+                                    287773481, 815094798, 356732984, 644469322, 543193620,
+                                    903158817, 274116865, 395252956, 363839119, 365378492,
+                                    122313059, 312690039, 252532812
+                                }),
+                equalTo(9));
+    }
+
+    @Test
+    void maxHappyGroups4() {
+        assertThat(
+                new Solution()
+                        .maxHappyGroups(
+                                9,
+                                new int[] {
+                                    3, 1, 3, 3, 5, 6, 1, 1, 9, 10, 3, 3, 3, 1, 1, 3, 3, 3, 19, 20,
+                                    1, 3, 3, 3, 3, 1, 1, 3, 3, 30
+                                }),
+                equalTo(9));
+    }
+
+    @Test
+    void maxHappyGroups5() {
+        assertThat(new Solution().maxHappyGroups(1, new int[] {1, 2, 3}), equalTo(3));
+    }
 }