Added 89

Author: trung-hn

Readme.md

@@ -74,6 +74,7 @@
 | 82   | [Remove Duplicates from Sorted List II](src/82.remove-duplicates-from-sorted-list-ii.py)                                                                             | Medium | O(N)                  | O(1)           | Linked List                                                                                          |                                         |           |
 | 84   | [Largest Rectangle in Histogram](src/84.largest-rectangle-in-histogram.py)                                                                                           | Hard   | O(N)                  | O(N)           | Array, Stack, Monotonic Stack                                                                        |                                         |           |
 | 85   | [Maximal Rectangle](src/85.maximal-rectangle.py)                                                                                                                     | Hard   | O(R\*C\*C)            | O(C)           | Array, Dynamic Programming, Stack, Matrix, Monotonic Stack                                           |                                         |           |
+| 89   | [Gray Code](src/89.gray-code.py)                                                                                                                                     | Medium | O(2^N)                | O(2^N)         | Math, Backtracking, Bit Manipulation                                                                 |                                         |           |
 | 91   | [Decode Ways](src/91.decode-ways.py)                                                                                                                                 | Medium | O(N)                  | O(N)           | String, Dynamic Programming                                                                          |                                         |           |
 | 92   | [Reverse Linked List II](src/92.reverse-linked-list-ii.py)                                                                                                           | Medium | O(N)                  | O(1)           | Linked List                                                                                          |                                         |           |
 | 94   | [Binary Tree Inorder Traversal](src/94.binary-tree-inorder-traversal.py)                                                                                             | Easy   | O(N)                  | O(N)           | Stack, Tree, DFS, Binary Tree                                                                        |                                         |           |

src/89.gray-code.py

@@ -0,0 +1,21 @@
+#
+# @lc app=leetcode id=89 lang=python3
+#
+# [89] Gray Code
+#
+
+
+# @lc code=start
+# TAGS: Math, Backtracking, Bit Manipulation
+class Solution:
+    # Time and Space: O(2^N)
+    def grayCode(self, n: int) -> List[int]:
+        ans = [0]
+        for i in range(n):
+            highest_bit = 1 << i
+            for j in reversed(range(highest_bit)):
+                ans.append(ans[j] + highest_bit)
+        return ans
+
+
+# @lc code=end