Add solutions for "Valid Palindrome" and "Top K Frequent Elements", update README

Author: hantdev

README.md

@@ -7,4 +7,6 @@
 |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 |
+|0049|[Group Anagrams](https://leetcode.com/problems/group-anagrams/description/) | [Python](./array_hashing/0049-group-anagrams.py)|Array & Hashing| Medium |
+|0125|[Valid Palindrome](https://leetcode.com/problems/valid-palindrome/description/) | [Python](./two_pointers/0125-valid-palindrome.py)|Two Pointers| Easy |
+|0347|[Top K Frequent Elements](https://leetcode.com/problems/top-k-frequent-elements/description/) | [Python](./array_hashing/0347-top-k-frequent-elements.py)|Array & Hashing| Medium |

array_hashing/0125-valid-palindrome.py

@@ -5,4 +5,18 @@ def topKFrequent(self, nums, k):
         :type k: int
         :rtype: List[int]
         """
+        freq_map = {}
+
+        for num in nums:
+            if num not in freq_map:
+                freq_map[num] = 0
+            freq_map[num] += 1
+
+        sorted_items = sorted(freq_map.items(), key=lambda x: x[1], reverse=True)
+
+        res = [item[0] for item in sorted_items[:k]]
+
+        return res
+
+
 

array_hashing/0347-top-k-frequent-elements.py

@@ -0,0 +1,25 @@
+class Solution(object):
+    def isValid(self, s):
+        """
+        :type s: str
+        :rtype: bool
+        """
+        if(len(s) %2 != 0): return False
+
+        stack = []
+
+        for c in s:
+            if c in "{[(":
+                stack.append(c)
+            elif c in "}])":
+                if not stack: 
+                    return False
+                if (c == "}" and stack[-1] == "{") or \
+                   (c == "]" and stack[-1] == "[") or \
+                   (c == ")" and stack[-1] == "("):
+                        stack.pop()
+                else:
+                    return False
+
+        return len(stack) == 0
+        

stack/0020-valid-parentheses.py

@@ -0,0 +1,19 @@
+class Solution(object):
+    def isPalindrome(self, s):
+        """
+        :type s: str
+        :rtype: bool
+        """
+
+        # new_s = ""
+
+        # for c in s:
+        #     if (c.isalnum() ): new_s += c
+
+        # new_s = new_s.lower()
+
+        # return new_s == new_s[::-1]
+
+        filtered_s = [c.lower() for c in s if c.isalnum()]
+        return filtered_s == filtered_s[::-1]
+