Add solution #446

Author: JoshCrozier

README.md

@@ -354,6 +354,7 @@
 442|[Find All Duplicates in an Array](./0442-find-all-duplicates-in-an-array.js)|Medium|
 443|[String Compression](./0443-string-compression.js)|Medium|
 445|[Add Two Numbers II](./0445-add-two-numbers-ii.js)|Medium|
+446|[Arithmetic Slices II - Subsequence](./0446-arithmetic-slices-ii-subsequence.js)|Hard|
 448|[Find All Numbers Disappeared in an Array](./0448-find-all-numbers-disappeared-in-an-array.js)|Easy|
 450|[Delete Node in a BST](./0450-delete-node-in-a-bst.js)|Medium|
 451|[Sort Characters By Frequency](./0451-sort-characters-by-frequency.js)|Medium|

solutions/0446-arithmetic-slices-ii-subsequence.js

@@ -0,0 +1,41 @@
+/**
+ * 446. Arithmetic Slices II - Subsequence
+ * https://leetcode.com/problems/arithmetic-slices-ii-subsequence/
+ * Difficulty: Hard
+ *
+ * Given an integer array nums, return the number of all the arithmetic subsequences of nums.
+ *
+ * A sequence of numbers is called arithmetic if it consists of at least three elements and if
+ * the difference between any two consecutive elements is the same.
+ *
+ * - For example, [1, 3, 5, 7, 9], [7, 7, 7, 7], and [3, -1, -5, -9] are arithmetic sequences.
+ * - For example, [1, 1, 2, 5, 7] is not an arithmetic sequence.
+ *
+ * A subsequence of an array is a sequence that can be formed by removing some elements (possibly
+ * none) of the array.
+ *
+ * - For example, [2,5,10] is a subsequence of [1,2,1,2,4,1,5,10].
+ *
+ * The test cases are generated so that the answer fits in 32-bit integer.
+ */
+
+/**
+ * @param {number[]} nums
+ * @return {number}
+ */
+var numberOfArithmeticSlices = function(nums) {
+  const dp = new Array(nums.length).fill().map(() => new Map());
+  let result = 0;
+
+  for (let i = 1; i < nums.length; i++) {
+    for (let j = 0; j < i; j++) {
+      const diff = nums[i] - nums[j];
+      const prev = dp[j].get(diff) || 0;
+      const current = (dp[i].get(diff) || 0) + prev + 1;
+      dp[i].set(diff, current);
+      result += prev;
+    }
+  }
+
+  return result;
+};