Add comprehensive LeetCode solutions index documentation

src/exercises/89.gray-code.py

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+from string import *
+from re import *
+from datetime import *
+from collections import *
+from heapq import *
+from bisect import *
+from copy import *
+from math import *
+from random import *
+from statistics import *
+from itertools import *
+from functools import *
+from operator import *
+from io import *
+from sys import *
+from json import *
+from builtins import *
+import string
+import re
+import datetime
+import collections
+import heapq
+import bisect
+import copy
+import math
+import random
+import statistics
+import itertools
+import functools
+import operator
+import io
+import sys
+import json
+from typing import *
+
+
+# @leet start
+class Solution:
+    def grayCode(self, n: int) -> List[int]:
+        return [i ^ (i >> 1) for i in range(1 << n)]
+        
+
+
+# @leet end
+