Add solution #3583

JoshCrozier · JoshCrozier · commit 143fcd7c1954 · 2025-12-09T16:54:36.000-06:00
diff --git a/README.md b/README.md
@@ -2786,6 +2786,7 @@
 3539|[Find Sum of Array Product of Magical Sequences](./solutions/3539-find-sum-of-array-product-of-magical-sequences.js)|Hard|
 3541|[Find Most Frequent Vowel and Consonant](./solutions/3541-find-most-frequent-vowel-and-consonant.js)|Easy|
 3578|[Count Partitions With Max-Min Difference at Most K](./solutions/3578-count-partitions-with-max-min-difference-at-most-k.js)|Medium|
+3583|[Count Special Triplets](./solutions/3583-count-special-triplets.js)|Medium|
 3623|[Count Number of Trapezoids I](./solutions/3623-count-number-of-trapezoids-i.js)|Medium|
 
 ## License
diff --git a/solutions/3583-count-special-triplets.js b/solutions/3583-count-special-triplets.js
@@ -0,0 +1,48 @@
+/**
+ * 3583. Count Special Triplets
+ * https://leetcode.com/problems/count-special-triplets/
+ * Difficulty: Medium
+ *
+ * You are given an integer array nums.
+ *
+ * A special triplet is defined as a triplet of indices (i, j, k) such that:
+ * - 0 <= i < j < k < n, where n = nums.length
+ * - nums[i] == nums[j] * 2
+ * - nums[k] == nums[j] * 2
+ *
+ * Return the total number of special triplets in the array.
+ *
+ * Since the answer may be large, return it modulo 109 + 7.
+ */
+
+/**
+ * @param {number[]} nums
+ * @return {number}
+ */
+var specialTriplets = function(nums) {
+  const MOD = 1e9 + 7;
+  let result = 0;
+
+  const leftCount = new Map();
+  const rightCount = new Map();
+  for (const num of nums) {
+    rightCount.set(num, (rightCount.get(num) || 0) + 1);
+  }
+
+  for (let j = 0; j < nums.length; j++) {
+    const middle = nums[j];
+    const target = middle * 2;
+
+    rightCount.set(middle, rightCount.get(middle) - 1);
+    if (rightCount.get(middle) === 0) {
+      rightCount.delete(middle);
+    }
+
+    const left = leftCount.get(target) || 0;
+    const right = rightCount.get(target) || 0;
+    result = (result + left * right) % MOD;
+    leftCount.set(middle, (leftCount.get(middle) || 0) + 1);
+  }
+
+  return result;
+};
