[Math] Add a solution to Gary Code

Math/GaryCode.swift

@@ -0,0 +1,18 @@
+/**
+ * Question Link: https://leetcode.com/problems/gray-code/
+ * Primary idea: the result of n can be derived from (n - 1) by reversing
+                 the order and prepending 1 to each number's binary representation
+ *
+ * Time Complexity: O(n), Space Complexity: O(2^n)
+ *
+ */
+
+class GaryCode {
+    func grayCode(_ n: Int) -> [Int] {
+        var codes = [0]
+        for i in 0..<n {
+            codes += codes.reversed().map { $0 | 1 << i }
+        }
+        return codes
+    }
+}

README.md

@@ -83,7 +83,7 @@
 [Gas Station](https://leetcode.com/problems/gas-station/)| [Swift](./Array/GasStation.swift)| Medium| O(n)| O(1)|
 [Game of Life](https://leetcode.com/problems/game-of-life/)| [Swift](./Array/GameLife.swift)| Medium| O(n)| O(1)|
 [Task Scheduler](https://leetcode.com/problems/task-scheduler/)| [Swift](./Array/TaskScheduler.swift)| Medium| O(nlogn)| O(n)|
-[Sliding Window Maximum ](https://leetcode.com/problems/sliding-window-maximum/)| [Swift](./Array/SlidingWindowMaximum.swift)| Hard| O(n)| O(n)|	
+[Sliding Window Maximum ](https://leetcode.com/problems/sliding-window-maximum/)| [Swift](./Array/SlidingWindowMaximum.swift)| Hard| O(n)| O(n)|
 [Longest Consecutive Sequence](https://leetcode.com/problems/longest-consecutive-sequence/)| [Swift](./Array/LongestConsecutiveSequence.swift)| Hard| O(n)| O(n)|
 [Create Maximum Number](https://leetcode.com/problems/create-maximum-number/)| [Swift](./Array/CreateMaximumNumber.swift)| Hard| O(n^2)| O(n)|
 
@@ -745,9 +745,9 @@
 |	[Swift](./Stack/InorderTraversal.swift)	|	94	|	[Binary Tree Inorder Traversal](https://oj.leetcode.com/problems/binary-tree-inorder-traversal/)	| Medium	|
 |		|	93	|	[Restore IP Addresses](https://oj.leetcode.com/problems/restore-ip-addresses/)	| Medium	|
 |		|	92	|	[Reverse Linked List II](https://oj.leetcode.com/problems/reverse-linked-list-ii/)	| Medium	|
-|	(./DP/DecodeWays.swift)	|	91	|	[Decode Ways](https://oj.leetcode.com/problems/decode-ways/)	| Medium	|
+|	[Swift](./DP/DecodeWays.swift)	|	91	|	[Decode Ways](https://oj.leetcode.com/problems/decode-ways/)	| Medium	|
 |	[Swift](./DFS/SubsetsII.swift)	|	90	|	[Subsets II](https://oj.leetcode.com/problems/subsets-ii/)	| Medium	|
-|		|	89	|	[Gray Code](https://oj.leetcode.com/problems/gray-code/)	| Medium	|
+|	[Swift](./Math/GaryCode.swift)	|	89	|	[Gray Code](https://oj.leetcode.com/problems/gray-code/)	| Medium	|
 |	[Swift](./Sort/MergeSortedArray.swift) |	88	|	[Merge Sorted Array](https://oj.leetcode.com/problems/merge-sorted-array/)	| Easy	|
 |		|	87	|	[Scramble String](https://oj.leetcode.com/problems/scramble-string/)	| Hard	|
 |	[Swift](./LinkedList/PartitionList.swift)	|	86	|	[Partition List](https://oj.leetcode.com/problems/partition-list/)	| Medium	|