Add solution #287

Author: JoshCrozier

README.md

@@ -226,6 +226,7 @@
 279|[Perfect Squares](./0279-perfect-squares.js)|Medium|
 282|[Expression Add Operators](./0282-expression-add-operators.js)|Hard|
 283|[Move Zeroes](./0283-move-zeroes.js)|Easy|
+287|[Find the Duplicate Number](./0287-find-the-duplicate-number.js)|Medium|
 289|[Game of Life](./0289-game-of-life.js)|Medium|
 290|[Word Pattern](./0290-word-pattern.js)|Easy|
 292|[Nim Game](./0292-nim-game.js)|Easy|

solutions/0287-find-the-duplicate-number.js

@@ -0,0 +1,33 @@
+/**
+ * 287. Find the Duplicate Number
+ * https://leetcode.com/problems/find-the-duplicate-number/
+ * Difficulty: Medium
+ *
+ * Given an array of integers nums containing n + 1 integers where each integer is in the
+ * range [1, n] inclusive.
+ *
+ * There is only one repeated number in nums, return this repeated number.
+ *
+ * You must solve the problem without modifying the array nums and using only constant extra space.
+ */
+
+/**
+ * @param {number[]} nums
+ * @return {number}
+ */
+var findDuplicate = function(nums) {
+  let slow = nums[0];
+  let fast = nums[nums[0]];
+
+  while (slow !== fast) {
+    slow = nums[slow];
+    fast = nums[nums[fast]];
+  }
+  slow = 0;
+  while (slow !== fast) {
+    slow = nums[slow];
+    fast = nums[fast];
+  }
+
+  return slow;
+};