Add solution #634

Author: JoshCrozier

README.md

@@ -589,6 +589,7 @@
 631|[Design Excel Sum Formula](./solutions/0631-design-excel-sum-formula.js)|Hard|
 632|[Smallest Range Covering Elements from K Lists](./solutions/0632-smallest-range-covering-elements-from-k-lists.js)|Hard|
 633|[Sum of Square Numbers](./solutions/0633-sum-of-square-numbers.js)|Medium|
+634|[Find the Derangement of An Array](./solutions/0634-find-the-derangement-of-an-array.js)|Medium|
 636|[Exclusive Time of Functions](./solutions/0636-exclusive-time-of-functions.js)|Medium|
 637|[Average of Levels in Binary Tree](./solutions/0637-average-of-levels-in-binary-tree.js)|Easy|
 638|[Shopping Offers](./solutions/0638-shopping-offers.js)|Medium|

solutions/0634-find-the-derangement-of-an-array.js

@@ -0,0 +1,32 @@
+/**
+ * 634. Find the Derangement of An Array
+ * https://leetcode.com/problems/find-the-derangement-of-an-array/
+ * Difficulty: Medium
+ *
+ * In combinatorial mathematics, a derangement is a permutation of the elements of a set,
+ * such that no element appears in its original position.
+ *
+ * You are given an integer n. There is originally an array consisting of n integers from
+ * 1 to n in ascending order, return the number of derangements it can generate. Since the
+ * answer may be huge, return it modulo 109 + 7.
+ */
+
+/**
+ * @param {number} n
+ * @return {number}
+ */
+var findDerangement = function(n) {
+  const MOD = 1e9 + 7;
+  if (n === 1) return 0;
+  if (n === 2) return 1;
+
+  let prev2 = 0;
+  let prev1 = 1;
+  for (let i = 3; i <= n; i++) {
+    const current = ((i - 1) * (prev1 + prev2)) % MOD;
+    prev2 = prev1;
+    prev1 = current;
+  }
+
+  return prev1;
+};