diff --git a/src/main/java/g3701_3800/s3780_maximum_sum_of_three_numbers_divisible_by_three/Solution.java b/src/main/java/g3701_3800/s3780_maximum_sum_of_three_numbers_divisible_by_three/Solution.java
new file mode 100644
index 000000000..44c78a026
--- /dev/null
+++ b/src/main/java/g3701_3800/s3780_maximum_sum_of_three_numbers_divisible_by_three/Solution.java
@@ -0,0 +1,51 @@
+package g3701_3800.s3780_maximum_sum_of_three_numbers_divisible_by_three;
+
+// #Medium #Array #Sorting #Greedy #Heap_Priority_Queue #Senior #Biweekly_Contest_172
+// #2026_05_06_Time_7_ms_(98.17%)_Space_168.14_MB_(89.91%)
+
+public class Solution {
+    public int maximumSum(int[] nums) {
+        int[][] group = new int[3][3];
+        for (int num : nums) {
+            int m = num % 3;
+            int max1 = group[m][0];
+            int max2 = group[m][1];
+            int max3 = group[m][2];
+            if (num >= max1) {
+                max3 = max2;
+                max2 = max1;
+                max1 = num;
+            } else if (num >= max2) {
+                max3 = max2;
+                max2 = num;
+            } else if (num > max3) {
+                max3 = num;
+            }
+            group[m][0] = max1;
+            group[m][1] = max2;
+            group[m][2] = max3;
+        }
+        int res = 0;
+        for (int[] g : group) {
+            int sum = 0;
+            for (int i = 0; i < 3; i += 1) {
+                if (g[i] == 0) {
+                    sum = 0;
+                    break;
+                }
+                sum += g[i];
+            }
+            res = Math.max(res, sum);
+        }
+        int max = 0;
+        for (int[] g : group) {
+            if (g[0] == 0) {
+                max = 0;
+                break;
+            }
+            max += g[0];
+        }
+        res = Math.max(res, max);
+        return res;
+    }
+}
diff --git a/src/main/java/g3701_3800/s3780_maximum_sum_of_three_numbers_divisible_by_three/readme.md b/src/main/java/g3701_3800/s3780_maximum_sum_of_three_numbers_divisible_by_three/readme.md
new file mode 100644
index 000000000..fce6203bd
--- /dev/null
+++ b/src/main/java/g3701_3800/s3780_maximum_sum_of_three_numbers_divisible_by_three/readme.md
@@ -0,0 +1,39 @@
+3780\. Maximum Sum of Three Numbers Divisible by Three
+
+Medium
+
+You are given an integer array `nums`.
+
+Your task is to choose **exactly three** integers from `nums` such that their sum is divisible by three.
+
+Return the **maximum** possible sum of such a triplet. If no such triplet exists, return 0.
+
+**Example 1:**
+
+**Input:** nums = [4,2,3,1]
+
+**Output:** 9
+
+**Explanation:**
+
+The valid triplets whose sum is divisible by 3 are:
+
+*   `(4, 2, 3)` with a sum of `4 + 2 + 3 = 9`.
+*   `(2, 3, 1)` with a sum of `2 + 3 + 1 = 6`.
+
+Thus, the answer is 9.
+
+**Example 2:**
+
+**Input:** nums = [2,1,5]
+
+**Output:** 0
+
+**Explanation:**
+
+No triplet forms a sum divisible by 3, so the answer is 0.
+
+**Constraints:**
+
+*   <code>3 <= nums.length <= 10<sup>5</sup></code>
+*   <code>1 <= nums[i] <= 10<sup>5</sup></code>
\ No newline at end of file
diff --git a/src/main/java/g3701_3800/s3781_maximum_score_after_binary_swaps/Solution.java b/src/main/java/g3701_3800/s3781_maximum_score_after_binary_swaps/Solution.java
new file mode 100644
index 000000000..5029d3fec
--- /dev/null
+++ b/src/main/java/g3701_3800/s3781_maximum_score_after_binary_swaps/Solution.java
@@ -0,0 +1,26 @@
+package g3701_3800.s3781_maximum_score_after_binary_swaps;
+
+// #Medium #Array #String #Greedy #Heap_Priority_Queue #Staff #Biweekly_Contest_172
+// #2026_05_06_Time_39_ms_(96.47%)_Space_118.00_MB_(29.41%)
+
+import java.util.PriorityQueue;
+
+public class Solution {
+    public long maximumScore(int[] nums, String s) {
+        long sum = 0;
+        PriorityQueue<Integer> pq = new PriorityQueue<>((a, b) -> b - a);
+        for (int i = 0; i < nums.length; i++) {
+            if (s.charAt(i) == '1') {
+                if (pq.isEmpty() || pq.peek() <= nums[i]) {
+                    sum += nums[i];
+                } else {
+                    sum += pq.poll();
+                    pq.add(nums[i]);
+                }
+            } else {
+                pq.add(nums[i]);
+            }
+        }
+        return sum;
+    }
+}
diff --git a/src/main/java/g3701_3800/s3781_maximum_score_after_binary_swaps/readme.md b/src/main/java/g3701_3800/s3781_maximum_score_after_binary_swaps/readme.md
new file mode 100644
index 000000000..af9c40b40
--- /dev/null
+++ b/src/main/java/g3701_3800/s3781_maximum_score_after_binary_swaps/readme.md
@@ -0,0 +1,43 @@
+3781\. Maximum Score After Binary Swaps
+
+Medium
+
+You are given an integer array `nums` of length `n` and a binary string `s` of the same length.
+
+Initially, your score is 0. Each index `i` where `s[i] = '1'` contributes `nums[i]` to the score.
+
+You may perform **any** number of operations (including zero). In one operation, you may choose an index `i` such that `0 <= i < n - 1`, where `s[i] = '0'`, and `s[i + 1] = '1'`, and swap these two characters.
+
+Return an integer denoting the **maximum possible score** you can achieve.
+
+**Example 1:**
+
+**Input:** nums = [2,1,5,2,3], s = "01010"
+
+**Output:** 7
+
+**Explanation:**
+
+We can perform the following swaps:
+
+*   Swap at index `i = 0`: `"01010"` changes to `"10010"`
+*   Swap at index `i = 2`: `"10010"` changes to `"10100"`
+
+Positions 0 and 2 contain `'1'`, contributing `nums[0] + nums[2] = 2 + 5 = 7`. This is maximum score achievable.
+
+**Example 2:**
+
+**Input:** nums = [4,7,2,9], s = "0000"
+
+**Output:** 0
+
+**Explanation:**
+
+There are no `'1'` characters in `s`, so no swaps can be performed. The score remains 0.
+
+**Constraints:**
+
+*   `n == nums.length == s.length`
+*   <code>1 <= n <= 10<sup>5</sup></code>
+*   <code>1 <= nums[i] <= 10<sup>9</sup></code>
+*   `s[i]` is either `'0'` or `'1'`
\ No newline at end of file
diff --git a/src/main/java/g3701_3800/s3782_last_remaining_integer_after_alternating_deletion_operations/Solution.java b/src/main/java/g3701_3800/s3782_last_remaining_integer_after_alternating_deletion_operations/Solution.java
new file mode 100644
index 000000000..7da4eac23
--- /dev/null
+++ b/src/main/java/g3701_3800/s3782_last_remaining_integer_after_alternating_deletion_operations/Solution.java
@@ -0,0 +1,11 @@
+package g3701_3800.s3782_last_remaining_integer_after_alternating_deletion_operations;
+
+// #Hard #Math #Recursion #Senior_Staff #Biweekly_Contest_172
+// #2026_05_06_Time_1_ms_(100.00%)_Space_43.05_MB_(9.38%)
+
+public class Solution {
+    public long lastInteger(long n) {
+        final long mask = 0xAAAAAAAAAAAAAAAL;
+        return ((n - 1) & mask) + 1;
+    }
+}
diff --git a/src/main/java/g3701_3800/s3782_last_remaining_integer_after_alternating_deletion_operations/readme.md b/src/main/java/g3701_3800/s3782_last_remaining_integer_after_alternating_deletion_operations/readme.md
new file mode 100644
index 000000000..7278cda22
--- /dev/null
+++ b/src/main/java/g3701_3800/s3782_last_remaining_integer_after_alternating_deletion_operations/readme.md
@@ -0,0 +1,53 @@
+3782\. Last Remaining Integer After Alternating Deletion Operations
+
+Hard
+
+You are given an integer `n`.
+
+We write the integers from 1 to `n` in a sequence from left to right. Then, **alternately** apply the following two operations until only one integer remains, starting with operation 1:
+
+*   **Operation 1**: Starting from the left, delete every second number.
+*   **Operation 2**: Starting from the right, delete every second number.
+
+Return the last remaining integer.
+
+**Example 1:**
+
+**Input:** n = 8
+
+**Output:** 3
+
+**Explanation:**
+
+*   Write `[1, 2, 3, 4, 5, 6, 7, 8]` in a sequence.
+*   Starting from the left, we delete every second number: <code>[1, <ins>**2**</ins>, 3, <ins>**4**</ins>, 5, <ins>**6**</ins>, 7, <ins>**8**</ins>]</code>. The remaining integers are `[1, 3, 5, 7]`.
+*   Starting from the right, we delete every second number: <code>[<ins>**1**</ins>, 3, <ins>**5**</ins>, 7]</code>. The remaining integers are `[3, 7]`.
+*   Starting from the left, we delete every second number: <code>[3, <ins>**7**</ins>]</code>. The remaining integer is `[3]`.
+
+**Example 2:**
+
+**Input:** n = 5
+
+**Output:** 1
+
+**Explanation:**
+
+*   Write `[1, 2, 3, 4, 5]` in a sequence.
+*   Starting from the left, we delete every second number: <code>[1, <ins>**2**</ins>, 3, <ins>**4**</ins>, 5]</code>. The remaining integers are `[1, 3, 5]`.
+*   Starting from the right, we delete every second number: <code>[1, <ins>**3**</ins>, 5]</code>. The remaining integers are `[1, 5]`.
+*   Starting from the left, we delete every second number: <code>[1, <ins>**5**</ins>]</code>. The remaining integer is `[1]`.
+
+**Example 3:**
+
+**Input:** n = 1
+
+**Output:** 1
+
+**Explanation:**
+
+*   Write `[1]` in a sequence.
+*   The last remaining integer is 1.
+
+**Constraints:**
+
+*   <code>1 <= n <= 10<sup>15</sup></code>
\ No newline at end of file
diff --git a/src/test/java/g3701_3800/s3780_maximum_sum_of_three_numbers_divisible_by_three/SolutionTest.java b/src/test/java/g3701_3800/s3780_maximum_sum_of_three_numbers_divisible_by_three/SolutionTest.java
new file mode 100644
index 000000000..54a0a6a6d
--- /dev/null
+++ b/src/test/java/g3701_3800/s3780_maximum_sum_of_three_numbers_divisible_by_three/SolutionTest.java
@@ -0,0 +1,18 @@
+package g3701_3800.s3780_maximum_sum_of_three_numbers_divisible_by_three;
+
+import static org.hamcrest.CoreMatchers.equalTo;
+import static org.hamcrest.MatcherAssert.assertThat;
+
+import org.junit.jupiter.api.Test;
+
+class SolutionTest {
+    @Test
+    void maximumSum() {
+        assertThat(new Solution().maximumSum(new int[] {4, 2, 3, 1}), equalTo(9));
+    }
+
+    @Test
+    void maximumSum2() {
+        assertThat(new Solution().maximumSum(new int[] {2, 1, 5}), equalTo(0));
+    }
+}
diff --git a/src/test/java/g3701_3800/s3781_maximum_score_after_binary_swaps/SolutionTest.java b/src/test/java/g3701_3800/s3781_maximum_score_after_binary_swaps/SolutionTest.java
new file mode 100644
index 000000000..dbe6b4895
--- /dev/null
+++ b/src/test/java/g3701_3800/s3781_maximum_score_after_binary_swaps/SolutionTest.java
@@ -0,0 +1,18 @@
+package g3701_3800.s3781_maximum_score_after_binary_swaps;
+
+import static org.hamcrest.CoreMatchers.equalTo;
+import static org.hamcrest.MatcherAssert.assertThat;
+
+import org.junit.jupiter.api.Test;
+
+class SolutionTest {
+    @Test
+    void maximumScore() {
+        assertThat(new Solution().maximumScore(new int[] {2, 1, 5, 2, 3}, "01010"), equalTo(7L));
+    }
+
+    @Test
+    void maximumScore2() {
+        assertThat(new Solution().maximumScore(new int[] {4, 7, 2, 9}, "0000"), equalTo(0L));
+    }
+}
diff --git a/src/test/java/g3701_3800/s3782_last_remaining_integer_after_alternating_deletion_operations/SolutionTest.java b/src/test/java/g3701_3800/s3782_last_remaining_integer_after_alternating_deletion_operations/SolutionTest.java
new file mode 100644
index 000000000..6ac50cda0
--- /dev/null
+++ b/src/test/java/g3701_3800/s3782_last_remaining_integer_after_alternating_deletion_operations/SolutionTest.java
@@ -0,0 +1,23 @@
+package g3701_3800.s3782_last_remaining_integer_after_alternating_deletion_operations;
+
+import static org.hamcrest.CoreMatchers.equalTo;
+import static org.hamcrest.MatcherAssert.assertThat;
+
+import org.junit.jupiter.api.Test;
+
+class SolutionTest {
+    @Test
+    void lastInteger() {
+        assertThat(new Solution().lastInteger(8L), equalTo(3L));
+    }
+
+    @Test
+    void lastInteger2() {
+        assertThat(new Solution().lastInteger(5L), equalTo(1L));
+    }
+
+    @Test
+    void lastInteger3() {
+        assertThat(new Solution().lastInteger(1L), equalTo(1L));
+    }
+}