Added task 3748

src/main/java/g3701_3800/s3748_count_stable_subarrays/Solution.java

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+package g3701_3800.s3748_count_stable_subarrays;
+
+// #Hard #Array #Binary_Search #Prefix_Sum #Senior_Staff #Weekly_Contest_476
+// #2026_04_26_Time_9_ms_(100.00%)_Space_183.68_MB_(43.30%)
+
+public class Solution {
+    public long[] countStableSubarrays(int[] nums, int[][] queries) {
+        int n = nums.length;
+        long[] preSum = new long[n + 1];
+        int[] idx = new int[n];
+        int[] end = new int[n];
+        int cnt = 0;
+        int prv = -1;
+        for (int i = 0; i < n; i++) {
+            if (nums[i] >= prv) {
+                cnt++;
+            } else {
+                cnt = 1;
+            }
+            prv = nums[i];
+            preSum[i + 1] = preSum[i] + cnt;
+            idx[i] = cnt - 1;
+        }
+        end[n - 1] = n - 1;
+        for (int i = n - 2; i >= 0; i--) {
+            if (idx[i] + 1 == idx[i + 1]) {
+                end[i] = end[i + 1];
+            } else {
+                end[i] = i;
+            }
+        }
+        long[] ans = new long[queries.length];
+        for (int l = 0; l < queries.length; l++) {
+            int i = queries[l][0];
+            int j = queries[l][1];
+            long res = preSum[j + 1] - preSum[i];
+            int endIdx = Math.min(end[i], j);
+            res -= (long) (endIdx - i + 1) * idx[i];
+            ans[l] = res;
+        }
+        return ans;
+    }
+}

src/main/java/g3701_3800/s3748_count_stable_subarrays/readme.md

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+3748\. Count Stable Subarrays
+
+Hard
+
+You are given an integer array `nums`.
+
+A ****non-empty subarrays**** of `nums` is called **stable** if it contains **no inversions**, i.e., there is no pair of indices `i < j` such that `nums[i] > nums[j]`.
+
+You are also given a **2D integer array** `queries` of length `q`, where each <code>queries[i] = [l<sub>i</sub>, r<sub>i</sub>]</code> represents a query. For each query <code>[l<sub>i</sub>, r<sub>i</sub>]</code>, compute the number of **stable subarrays** that lie entirely within the segment <code>nums[l<sub>i</sub>..r<sub>i</sub>]</code>.
+
+Return an integer array `ans` of length `q`, where `ans[i]` is the answer to the <code>i<sup>th</sup></code> query.
+
+**Note**:
+
+*   A single element subarray is considered stable.
+
+**Example 1:**
+
+**Input:** nums = [3,1,2], queries = [[0,1],[1,2],[0,2]]
+
+**Output:** [2,3,4]
+
+**Explanation:**
+
+*   For `queries[0] = [0, 1]`, the subarray is `[nums[0], nums[1]] = [3, 1]`.
+    *   The stable subarrays are `[3]` and `[1]`. The total number of stable subarrays is 2.
+*   For `queries[1] = [1, 2]`, the subarray is `[nums[1], nums[2]] = [1, 2]`.
+    *   The stable subarrays are `[1]`, `[2]`, and `[1, 2]`. The total number of stable subarrays is 3.
+*   For `queries[2] = [0, 2]`, the subarray is `[nums[0], nums[1], nums[2]] = [3, 1, 2]`.
+    *   The stable subarrays are `[3]`, `[1]`, `[2]`, and `[1, 2]`. The total number of stable subarrays is 4.
+
+Thus, `ans = [2, 3, 4]`.
+
+**Example 2:**
+
+**Input:** nums = [2,2], queries = [[0,1],[0,0]]
+
+**Output:** [3,1]
+
+**Explanation:**
+
+*   For `queries[0] = [0, 1]`, the subarray is `[nums[0], nums[1]] = [2, 2]`.
+    *   The stable subarrays are `[2]`, `[2]`, and `[2, 2]`. The total number of stable subarrays is 3.
+*   For `queries[1] = [0, 0]`, the subarray is `[nums[0]] = [2]`.
+    *   The stable subarray is `[2]`. The total number of stable subarrays is 1.
+
+Thus, `ans = [3, 1]`.
+
+**Constraints:**
+
+*   <code>1 <= nums.length <= 10<sup>5</sup></code>
+*   <code>1 <= nums[i] <= 10<sup>5</sup></code>
+*   <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 - 1</code>

src/test/java/g3701_3800/s3748_count_stable_subarrays/SolutionTest.java

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+package g3701_3800.s3748_count_stable_subarrays;
+
+import static org.hamcrest.CoreMatchers.equalTo;
+import static org.hamcrest.MatcherAssert.assertThat;
+
+import org.junit.jupiter.api.Test;
+
+class SolutionTest {
+    @Test
+    void countStableSubarrays() {
+        assertThat(
+                new Solution()
+                        .countStableSubarrays(
+                                new int[] {3, 1, 2}, new int[][] {{0, 1}, {1, 2}, {0, 2}}),
+                equalTo(new long[] {2L, 3L, 4L}));
+    }
+
+    @Test
+    void countStableSubarrays2() {
+        assertThat(
+                new Solution().countStableSubarrays(new int[] {2, 2}, new int[][] {{0, 1}, {0, 0}}),
+                equalTo(new long[] {3L, 1L}));
+    }
+}