diff --git a/src/main/java/g3901_4000/s3917_count_indices_with_opposite_parity/Solution.java b/src/main/java/g3901_4000/s3917_count_indices_with_opposite_parity/Solution.java
new file mode 100644
index 000000000..fb959f75b
--- /dev/null
+++ b/src/main/java/g3901_4000/s3917_count_indices_with_opposite_parity/Solution.java
@@ -0,0 +1,23 @@
+package g3901_4000.s3917_count_indices_with_opposite_parity;
+
+// #Easy #Array #Mid_Level #Weekly_Contest_500
+// #2026_09_15_Time_1_ms_(100.00%)_Space_46.74_MB_(66.61%)
+
+public class Solution {
+    public int[] countOppositeParity(int[] nums) {
+        int n = nums.length;
+        int odd = 0;
+        int even = 0;
+        int[] result = new int[n];
+        for (int i = n - 1; i >= 0; i--) {
+            if ((nums[i] & 1) == 1) {
+                result[i] = even;
+                odd++;
+            } else {
+                result[i] = odd;
+                even++;
+            }
+        }
+        return result;
+    }
+}
diff --git a/src/main/java/g3901_4000/s3917_count_indices_with_opposite_parity/readme.md b/src/main/java/g3901_4000/s3917_count_indices_with_opposite_parity/readme.md
new file mode 100644
index 000000000..06118743d
--- /dev/null
+++ b/src/main/java/g3901_4000/s3917_count_indices_with_opposite_parity/readme.md
@@ -0,0 +1,42 @@
+3917\. Count Indices With Opposite Parity
+
+Easy
+
+You are given an integer array `nums` of length `n`.
+
+The **score** of an index `i` is defined as the number of indices `j` such that:
+
+*   `i < j < n`, and
+*   `nums[i]` and `nums[j]` have different parity (one is even and the other is odd).
+
+Return an integer array `answer` of length `n`, where `answer[i]` is the score of index `i`.
+
+**Example 1:**
+
+**Input:** nums = [1,2,3,4]
+
+**Output:** [2,1,1,0]
+
+**Explanation:**
+
+*   `nums[0] = 1`, which is odd. Thus, the indices `j = 1` and `j = 3` satisfy the conditions, so the score of index 0 is 2.
+*   `nums[1] = 2`, which is even. Thus, the index `j = 2` satisfies the conditions, so the score of index 1 is 1.
+*   `nums[2] = 3`, which is odd. Thus, the index `j = 3` satisfies the conditions, so the score of index 2 is 1.
+*   `nums[3] = 4`, which is even. Thus, no index satisfies the conditions, so the score of index 3 is 0.
+
+Thus, the `answer = [2, 1, 1, 0]`.
+
+**Example 2:**
+
+**Input:** nums = [1]
+
+**Output:** [0]
+
+**Explanation:**
+
+There is only one element in `nums`. Thus, the score of index 0 is 0.
+
+**Constraints:**
+
+*   `1 <= nums.length <= 100`
+*   `1 <= nums[i] <= 100`
\ No newline at end of file
diff --git a/src/main/java/g3901_4000/s3918_sum_of_primes_between_number_and_its_reverse/Solution.java b/src/main/java/g3901_4000/s3918_sum_of_primes_between_number_and_its_reverse/Solution.java
new file mode 100644
index 000000000..a557ad120
--- /dev/null
+++ b/src/main/java/g3901_4000/s3918_sum_of_primes_between_number_and_its_reverse/Solution.java
@@ -0,0 +1,46 @@
+package g3901_4000.s3918_sum_of_primes_between_number_and_its_reverse;
+
+// #Medium #Math #Number_Theory #Senior #Weekly_Contest_500
+// #2026_09_15_Time_3_ms_(96.45%)_Space_42.41_MB_(83.95%)
+
+public class Solution {
+    private boolean isPrime(int x) {
+        if (x <= 1) {
+            return false;
+        }
+        if (x == 2) {
+            return true;
+        }
+        if (x % 2 == 0) {
+            return false;
+        }
+        for (int i = 3; i * i <= x; i += 2) {
+            if (x % i == 0) {
+                return false;
+            }
+        }
+        return true;
+    }
+
+    private int reverseNum(int n) {
+        int r = 0;
+        while (n > 0) {
+            r = r * 10 + (n % 10);
+            n /= 10;
+        }
+        return r;
+    }
+
+    public int sumOfPrimesInRange(int n) {
+        int r = reverseNum(n);
+        int low = Math.min(n, r);
+        int high = Math.max(n, r);
+        int sum = 0;
+        for (int i = low; i <= high; i++) {
+            if (isPrime(i)) {
+                sum += i;
+            }
+        }
+        return sum;
+    }
+}
diff --git a/src/main/java/g3901_4000/s3918_sum_of_primes_between_number_and_its_reverse/readme.md b/src/main/java/g3901_4000/s3918_sum_of_primes_between_number_and_its_reverse/readme.md
new file mode 100644
index 000000000..4e73cd7c7
--- /dev/null
+++ b/src/main/java/g3901_4000/s3918_sum_of_primes_between_number_and_its_reverse/readme.md
@@ -0,0 +1,48 @@
+3918\. Sum of Primes Between Number and Its Reverse
+
+Medium
+
+You are given an integer `n`.
+
+Let `r` be the integer formed by reversing the digits of `n`.
+
+Return the **sum** of all prime numbers between `min(n, r)` and `max(n, r)`, inclusive.
+
+**Example 1:**
+
+**Input:** n = 13
+
+**Output:** 132
+
+**Explanation:**
+
+*   The reverse of 13 is 31. Thus, the range is `[13, 31]`.
+*   The prime numbers in this range are 13, 17, 19, 23, 29, and 31.
+*   The sum of these prime numbers is `13 + 17 + 19 + 23 + 29 + 31 = 132`.
+
+**Example 2:**
+
+**Input:** n = 10
+
+**Output:** 17
+
+**Explanation:**
+
+*   The reverse of 10 is 1. Thus, the range is `[1, 10]`.
+*   The prime numbers in this range are 2, 3, 5, and 7.
+*   The sum of these prime numbers is `2 + 3 + 5 + 7 = 17`.
+
+**Example 3:**
+
+**Input:** n = 8
+
+**Output:** 0
+
+**Explanation:**
+
+*   The reverse of 8 is 8. Thus, the range is `[8, 8]`.
+*   There are no prime numbers in this range, so the sum is 0.
+
+**Constraints:**
+
+*   `1 <= n <= 1000`
\ No newline at end of file
diff --git a/src/main/java/g3901_4000/s3919_minimum_cost_to_move_between_indices/Solution.java b/src/main/java/g3901_4000/s3919_minimum_cost_to_move_between_indices/Solution.java
new file mode 100644
index 000000000..8bd975594
--- /dev/null
+++ b/src/main/java/g3901_4000/s3919_minimum_cost_to_move_between_indices/Solution.java
@@ -0,0 +1,37 @@
+package g3901_4000.s3919_minimum_cost_to_move_between_indices;
+
+// #Medium #Array #Greedy #Prefix_Sum #Staff #Weekly_Contest_500
+// #2026_09_15_Time_4_ms_(100.00%)_Space_187.52_MB_(54.12%)
+
+public class Solution {
+    public int[] minCost(int[] nums, int[][] queries) {
+        int n = nums.length;
+        int[] prefixSum = new int[n];
+        int[] suffixSum = new int[n];
+        prefixSum[1] = 1;
+        for (int i = 1; i < n - 1; i++) {
+            int left = Math.abs(nums[i] - nums[i - 1]);
+            int right = Math.abs(nums[i] - nums[i + 1]);
+            if (left <= right) {
+                prefixSum[i + 1] = prefixSum[i] + right;
+                suffixSum[i] = suffixSum[i - 1] + 1;
+            } else {
+                prefixSum[i + 1] = prefixSum[i] + 1;
+                suffixSum[i] = suffixSum[i - 1] + left;
+            }
+        }
+        suffixSum[n - 1] = suffixSum[n - 2] + 1;
+        int[] ans = new int[queries.length];
+        int i = 0;
+        for (int[] qur : queries) {
+            int l = qur[0];
+            int r = qur[1];
+            if (l > r) {
+                ans[i++] = suffixSum[l] - suffixSum[r];
+            } else {
+                ans[i++] = prefixSum[r] - prefixSum[l];
+            }
+        }
+        return ans;
+    }
+}
diff --git a/src/main/java/g3901_4000/s3919_minimum_cost_to_move_between_indices/readme.md b/src/main/java/g3901_4000/s3919_minimum_cost_to_move_between_indices/readme.md
new file mode 100644
index 000000000..26a49ece6
--- /dev/null
+++ b/src/main/java/g3901_4000/s3919_minimum_cost_to_move_between_indices/readme.md
@@ -0,0 +1,59 @@
+3919\. Minimum Cost to Move Between Indices
+
+Medium
+
+You are given an integer array `nums` where `nums` is **strictly increasing**.
+
+For each index `x`, let `closest(x)` be the **adjacent** index `y` such that `abs(nums[x] - nums[y])` is **minimized**. If both **adjacent** indices exist and give the same difference, choose the **smaller** index.
+
+From any index `x`, you can move in two ways:
+
+*   To any index `y` with cost `abs(nums[x] - nums[y])`, or
+*   To `closest(x)` with cost 1.
+
+You are also given a 2D integer array `queries`, where each <code>queries[i] = [l<sub>i</sub>, r<sub>i</sub>]</code>.
+
+For each query, calculate the **minimum total cost** to move from index <code>l<sub>i</sub></code> to index <code>r<sub>i</sub></code>.
+
+Return an integer array `ans`, where `ans[i]` is the answer for the <code>i<sup>th</sup></code> query.
+
+The **absolute difference** between two values `x` and `y` is defined as `abs(x - y)`.
+
+**Example 1:**
+
+**Input:** nums = [-5,-2,3], queries = [[0,2],[2,0],[1,2]]
+
+**Output:** [6,2,5]
+
+**Explanation:**
+
+*   The closest indices are `[1, 0, 1]` respectively.
+*   For `[0, 2]`, the path `0 → 1 → 2` uses a closest move from index 0 to 1 with cost 1 and a move from index 1 to 2 with cost `|-2 - 3| = 5`, giving total `1 + 5 = 6`.
+*   For `[2, 0]`, the path `2 → 1 → 0` uses two closest moves from index 2 to 1 and from index 1 to 0, each with cost 1, giving total 2.
+*   For `[1, 2]`, the direct move from index 1 to index 2 has cost `|-2 - 3| = 5`, which is optimal.
+
+Thus, `ans = [6, 2, 5]`.
+
+**Example 2:**
+
+**Input:** nums = [0,2,3,9], queries = [[3,0],[1,2],[2,0]]
+
+**Output:** [4,1,3]
+
+**Explanation:**
+
+*   The closest indices are `[1, 2, 1, 2]` respectively.
+*   For `[3, 0]`, the path `3 → 2 → 1 → 0` uses closest moves from index 3 to 2 and from 2 to 1, each with cost 1, and a move from 1 to 0 with cost `|2 - 0| = 2`, giving total `1 + 1 + 2 = 4`.
+*   For `[1, 2]`, the closest move from index 1 to 2 has cost 1.
+*   For `[2, 0]`, the path `2 → 1 → 0` uses a closest move from index 2 to 1 with cost 1 and a move from 1 to 0 with cost `|2 - 0| = 2`, giving total `1 + 2 = 3`.
+
+Thus, `ans = [4, 1, 3]`.
+
+**Constraints:**
+
+*   <code>2 <= nums.length <= 10<sup>5</sup></code>
+*   <code>-10<sup>9</sup> <= nums[i] <= 10<sup>9</sup></code>
+*   `nums` is strictly increasing
+*   <code>1 <= queries.length <= 10<sup>5</sup></code>
+*   <code>queries[i] = [l<sub>i</sub>, r<sub>i</sub>]</code>
+*   <code>0 <= l<sub>i</sub>, r<sub>i</sub> < nums.length</code>
\ No newline at end of file
diff --git a/src/test/java/g3901_4000/s3917_count_indices_with_opposite_parity/SolutionTest.java b/src/test/java/g3901_4000/s3917_count_indices_with_opposite_parity/SolutionTest.java
new file mode 100644
index 000000000..92e6647c5
--- /dev/null
+++ b/src/test/java/g3901_4000/s3917_count_indices_with_opposite_parity/SolutionTest.java
@@ -0,0 +1,20 @@
+package g3901_4000.s3917_count_indices_with_opposite_parity;
+
+import static org.hamcrest.CoreMatchers.equalTo;
+import static org.hamcrest.MatcherAssert.assertThat;
+
+import org.junit.jupiter.api.Test;
+
+class SolutionTest {
+    @Test
+    void countOppositeParity() {
+        assertThat(
+                new Solution().countOppositeParity(new int[] {1, 2, 3, 4}),
+                equalTo(new int[] {2, 1, 1, 0}));
+    }
+
+    @Test
+    void countOppositeParity2() {
+        assertThat(new Solution().countOppositeParity(new int[] {1}), equalTo(new int[] {0}));
+    }
+}
diff --git a/src/test/java/g3901_4000/s3918_sum_of_primes_between_number_and_its_reverse/SolutionTest.java b/src/test/java/g3901_4000/s3918_sum_of_primes_between_number_and_its_reverse/SolutionTest.java
new file mode 100644
index 000000000..3af9381e8
--- /dev/null
+++ b/src/test/java/g3901_4000/s3918_sum_of_primes_between_number_and_its_reverse/SolutionTest.java
@@ -0,0 +1,23 @@
+package g3901_4000.s3918_sum_of_primes_between_number_and_its_reverse;
+
+import static org.hamcrest.CoreMatchers.equalTo;
+import static org.hamcrest.MatcherAssert.assertThat;
+
+import org.junit.jupiter.api.Test;
+
+class SolutionTest {
+    @Test
+    void sumOfPrimesInRange() {
+        assertThat(new Solution().sumOfPrimesInRange(13), equalTo(132));
+    }
+
+    @Test
+    void sumOfPrimesInRange2() {
+        assertThat(new Solution().sumOfPrimesInRange(10), equalTo(17));
+    }
+
+    @Test
+    void sumOfPrimesInRange3() {
+        assertThat(new Solution().sumOfPrimesInRange(8), equalTo(0));
+    }
+}
diff --git a/src/test/java/g3901_4000/s3919_minimum_cost_to_move_between_indices/SolutionTest.java b/src/test/java/g3901_4000/s3919_minimum_cost_to_move_between_indices/SolutionTest.java
new file mode 100644
index 000000000..6889afc70
--- /dev/null
+++ b/src/test/java/g3901_4000/s3919_minimum_cost_to_move_between_indices/SolutionTest.java
@@ -0,0 +1,23 @@
+package g3901_4000.s3919_minimum_cost_to_move_between_indices;
+
+import static org.hamcrest.CoreMatchers.equalTo;
+import static org.hamcrest.MatcherAssert.assertThat;
+
+import org.junit.jupiter.api.Test;
+
+class SolutionTest {
+    @Test
+    void minCost() {
+        assertThat(
+                new Solution().minCost(new int[] {-5, -2, 3}, new int[][] {{0, 2}, {2, 0}, {1, 2}}),
+                equalTo(new int[] {6, 2, 5}));
+    }
+
+    @Test
+    void minCost2() {
+        assertThat(
+                new Solution()
+                        .minCost(new int[] {0, 2, 3, 9}, new int[][] {{3, 0}, {1, 2}, {2, 0}}),
+                equalTo(new int[] {4, 1, 3}));
+    }
+}