Add solution for "Group Anagrams" and update README

Author: hantdev

README.md

@@ -7,3 +7,4 @@
 |0217|[Contains Duplicate](https://leetcode.com/problems/contains-duplicate/description/) | [Python](./array_hashing/0217-contains-duplicate.py)|Array & Hashing| Easy |
 |0242|[Valid Anagram](https://leetcode.com/problems/valid-anagram/description/) | [Python](./array_hashing/0242-valid-anagram.py)|Array & Hashing| Easy |
 |0001|[Two Sum](https://leetcode.com/problems/two-sum/description/) | [Python](./array_hashing/0001-two-sum.py)|Array & Hashing| Easy |
+|0049|[Group Anagrams](https://leetcode.com/problems/group-anagrams/description/) | [Python](./array_hashing/0049-group-anagrams.py)|Array & Hashing| Medium |

array_hashing/0049-group-anagrams.py

@@ -0,0 +1,25 @@
+class Solution(object):
+    def groupAnagrams(self, strs):
+        """
+        :type strs: List[str]
+        :rtype: List[List[str]]
+        """
+        anagrams_dict = defaultdict(list)
+
+        for s in strs: # n
+            count = [0] * 26 # a ... z
+
+            for c in s: # m
+                # Map a - 0, b - 1, ..., z - 25
+                count[ord(c) - ord("a")] += 1
+            # immutable -> hashable
+            key = tuple(count)
+            anagrams_dict[key].append(s)
+
+        # print(anagrams_dict)
+
+        return anagrams_dict.values()
+
+        # Time: O(n * m) Space: O(n * m)
+            
+        

array_hashing/0125-valid-palindrome.py

@@ -0,0 +1,8 @@
+class Solution(object):
+    def topKFrequent(self, nums, k):
+        """
+        :type nums: List[int]
+        :type k: int
+        :rtype: List[int]
+        """
+