feat: 2025-04-25 daily leetcode Count of Interesting Subarrays

leetcode/daily/2025-04-25-count-of-interesting-subarrays.js

@@ -0,0 +1,49 @@
+// Count of Interesting Subarrays
+// Created on 2025-04-25
+
+/**
+ * @param {number[]} nums
+ * @param {number} modulo
+ * @param {number} k
+ * @return {number}
+ */
+function solution(nums, modulo, k) {
+  //My soulution
+  // let count = 0;
+  // for (let i = 0; i < nums.length; i++) {
+  //   let cnt = 0;
+  //   for (let j = i; j < nums.length; j++) {
+  //     if (nums[j] % modulo === k) {
+  //       cnt++;
+  //     }
+  //     if (cnt % modulo === k) {
+  //       count++;
+  //     }
+  //   }
+  // }
+  // return count;
+
+  // Optimized solution
+  let count = 0;
+  let prefix = 0;
+  const map = new Map();
+  map.set(0, 1);
+
+  for (const num of nums) {
+    if (num % modulo === k) {
+      prefix += 1;
+    }
+    const remainder = prefix % modulo;
+    const want = (remainder - k + modulo) % modulo;
+
+    if (map.has(want)) {
+      count += map.get(want);
+    }
+
+    map.set(remainder, (map.get(remainder) || 0) + 1);
+  }
+
+  return count;
+}
+
+console.log(solution([3,2,4], 2, 1))

leetcode/daily/readme.md

@@ -1,13 +1,26 @@
 ## 🔍 Daily LeetCode Challenges
 
-![Challenges Solved](https://img.shields.io/badge/Challenges%20Solved-1-blue)
-![Last Update](https://img.shields.io/badge/Last_Update-2025--04--21-success)
+
+![Challenges Solved](https://img.shields.io/badge/Challenges%20Solved-2-blue)
+![Last Update](https://img.shields.io/badge/Last_Update-2025--04--25-success)
+
 
 🧠 This folder contains **daily LeetCode challenges** I'm solving to improve my problem-solving and algorithmic thinking.  
 Each day I tackle a new problem and document my approach and solution.
 
 ### Challenge List
 <!-- NEW_CHALLENGES_HERE -->
+### [Count of Interesting Subarrays](https://leetcode.com/problems/count-of-interesting-subarrays/)
+- **Problem**: You are given a 0-indexed integer array `nums`, an integer `modulo`, and an integer `k`.
+  Your task is to find the count of subarrays that are interesting.
+  A subarray `nums[l..r]` is interesting if the following condition holds:
+  Let `cnt` be the number of indices `i` in the range `[l, r]` such that `nums[i] % modulo == k`. Then, `cnt % modulo == k`.
+  Return an integer denoting the count of interesting subarrays.
+
+- **Approach**: A brute-force solution would involve checking all possible subarrays, which leads to time limit exceeded on large inputs. To optimize, I use a prefix count of how many values satisfy num % modulo === k and store the counts modulo modulo in a HashMap. This allows to calculate in O(n) time how many previous prefix counts make the current one satisfy the condition using modular arithmetic and prefix sums.
+
+- **Solution File**: `daily/2025-04-25-count-of-interesting-subarrays.js`
+
 
 ### [Count the Hidden Sequences](https://leetcode.com/problems/count-the-hidden-sequences/)
 - **Problem**: Description of the problem goes here.