Add solution #629

Author: JoshCrozier

README.md

@@ -475,6 +475,7 @@
 623|[Add One Row to Tree](./0623-add-one-row-to-tree.js)|Medium|
 624|[Maximum Distance in Arrays](./0624-maximum-distance-in-arrays.js)|Medium|
 628|[Maximum Product of Three Numbers](./0628-maximum-product-of-three-numbers.js)|Easy|
+629|[K Inverse Pairs Array](./0629-k-inverse-pairs-array.js)|Hard|
 637|[Average of Levels in Binary Tree](./0637-average-of-levels-in-binary-tree.js)|Easy|
 643|[Maximum Average Subarray I](./0643-maximum-average-subarray-i.js)|Easy|
 645|[Set Mismatch](./0645-set-mismatch.js)|Medium|

solutions/0629-k-inverse-pairs-array.js

@@ -0,0 +1,34 @@
+/**
+ * 629. K Inverse Pairs Array
+ * https://leetcode.com/problems/k-inverse-pairs-array/
+ * Difficulty: Hard
+ *
+ * For an integer array nums, an inverse pair is a pair of integers [i, j] where
+ * 0 <= i < j < nums.length and nums[i] > nums[j].
+ *
+ * Given two integers n and k, return the number of different arrays consisting of numbers from
+ * 1 to n such that there are exactly k inverse pairs. Since the answer can be huge, return
+ * it modulo 109 + 7.
+ */
+
+/**
+ * @param {number} n
+ * @param {number} k
+ * @return {number}
+ */
+var kInversePairs = function(n, k) {
+  const MOD = 1e9 + 7;
+  const dp = new Array(k + 1).fill(0);
+
+  dp[0] = 1;
+  for (let i = 1; i <= n; i++) {
+    const previous = dp.slice();
+    dp[0] = 1;
+    for (let j = 1; j <= k; j++) {
+      dp[j] = (previous[j] + dp[j-1]) % MOD;
+      if (j >= i) dp[j] = (dp[j] - previous[j-i] + MOD) % MOD;
+    }
+  }
+
+  return dp[k];
+};