Add solution #923

Author: JoshCrozier

README.md

@@ -732,6 +732,7 @@
 920|[Number of Music Playlists](./solutions/0920-number-of-music-playlists.js)|Hard|
 921|[Minimum Add to Make Parentheses Valid](./solutions/0921-minimum-add-to-make-parentheses-valid.js)|Medium|
 922|[Sort Array By Parity II](./solutions/0922-sort-array-by-parity-ii.js)|Easy|
+923|[3Sum With Multiplicity](./solutions/0923-3sum-with-multiplicity.js)|Medium|
 925|[Long Pressed Name](./solutions/0925-long-pressed-name.js)|Easy|
 926|[Flip String to Monotone Increasing](./solutions/0926-flip-string-to-monotone-increasing.js)|Medium|
 929|[Unique Email Addresses](./solutions/0929-unique-email-addresses.js)|Easy|

solutions/0923-3sum-with-multiplicity.js

@@ -0,0 +1,53 @@
+/**
+ * 923. 3Sum With Multiplicity
+ * https://leetcode.com/problems/3sum-with-multiplicity/
+ * Difficulty: Medium
+ *
+ * Given an integer array arr, and an integer target, return the number of tuples i, j, k such
+ * that i < j < k and arr[i] + arr[j] + arr[k] == target.
+ *
+ * As the answer can be very large, return it modulo 109 + 7.
+ */
+
+/**
+ * @param {number[]} arr
+ * @param {number} target
+ * @return {number}
+ */
+var threeSumMulti = function(arr, target) {
+  const MOD = 1e9 + 7;
+  const counts = new Map();
+  let result = 0;
+
+  for (const num of arr) {
+    counts.set(num, (counts.get(num) || 0) + 1);
+  }
+
+  const uniqueNums = [...counts.keys()].sort((a, b) => a - b);
+
+  for (let i = 0; i < uniqueNums.length; i++) {
+    const num1 = uniqueNums[i];
+    for (let j = i; j < uniqueNums.length; j++) {
+      const num2 = uniqueNums[j];
+      const num3 = target - num1 - num2;
+
+      if (num3 < num2) continue;
+
+      const count1 = counts.get(num1);
+      const count2 = counts.get(num2);
+      const count3 = counts.get(num3) || 0;
+
+      if (num1 === num2 && num2 === num3) {
+        result = (result + (count1 * (count1 - 1) * (count1 - 2)) / 6) % MOD;
+      } else if (num1 === num2) {
+        result = (result + (count1 * (count1 - 1) * count3) / 2) % MOD;
+      } else if (num2 === num3) {
+        result = (result + (count1 * count2 * (count2 - 1)) / 2) % MOD;
+      } else {
+        result = (result + count1 * count2 * count3) % MOD;
+      }
+    }
+  }
+
+  return result;
+};