diff --git a/src/my_project/interviews/amazon_high_frequency_23/common_algos/two_sum_round_14.py b/src/my_project/interviews/amazon_high_frequency_23/common_algos/two_sum_round_14.py
new file mode 100644
index 00000000..6c0c6330
--- /dev/null
+++ b/src/my_project/interviews/amazon_high_frequency_23/common_algos/two_sum_round_14.py
@@ -0,0 +1,16 @@
+from typing import List, Union, Collection, Mapping, Optional
+from abc import ABC, abstractmethod
+
+class Solution:
+    def twoSum(self, nums: List[int], target: int) -> List[int]:
+
+        answer = dict()
+
+        for k, v in enumerate(nums):
+
+            if v in answer:
+                return [answer[v], k]
+            else:
+                answer[target - v] = k
+
+        return []
\ No newline at end of file
diff --git a/src/my_project/interviews/amazon_high_frequency_23/common_algos/valid_palindrome_round_14.py b/src/my_project/interviews/amazon_high_frequency_23/common_algos/valid_palindrome_round_14.py
new file mode 100644
index 00000000..14ef21aa
--- /dev/null
+++ b/src/my_project/interviews/amazon_high_frequency_23/common_algos/valid_palindrome_round_14.py
@@ -0,0 +1,23 @@
+from typing import List, Union, Collection, Mapping, Optional
+from abc import ABC, abstractmethod
+import re
+
+class Solution:
+    def isPalindrome(self, s: str) -> bool:
+
+        # To lower 
+        s = s.lower()
+
+        # Remove non-alphanumeric characters
+        s = re.sub(pattern=r'[^a-zA-Z0-9]', repl='', string=s)
+
+        # Determine if s is palindrome or not
+
+        len_s = len(s)
+
+        for i in range(len_s//2):
+
+            if s[i] != s[len_s - 1 - i]:
+                return False 
+            
+        return True 
\ No newline at end of file
diff --git a/src/my_project/interviews/amazon_high_frequency_23/round_4/longest_common_subsequence.py b/src/my_project/interviews/amazon_high_frequency_23/round_4/longest_common_subsequence.py
new file mode 100644
index 00000000..d5d4d0af
--- /dev/null
+++ b/src/my_project/interviews/amazon_high_frequency_23/round_4/longest_common_subsequence.py
@@ -0,0 +1,22 @@
+from typing import List, Union, Collection, Mapping, Optional
+
+class Solution:
+    def longestCommonSubsequence(self, text1: str, text2: str) -> int:
+
+        # Make a grid of 0's with len(text2) + 1 columns 
+        # and len(text1) + 1 rows.
+        len_1 = len(text1)
+        len_2 = len(text2)
+        dp_grid = [[0]*(len_2+1) for _ in range(len_1+1)]
+
+        # Iterate up each column, starting from the last one. 
+        for j in reversed(range(len_2)):
+            for i in reversed(range(len_1)):
+                if text1[i] == text2[j]:
+                    dp_grid[i][j] = dp_grid[i+1][j+1] + 1
+                else:
+                    dp_grid[i][j] = max(dp_grid[i+1][j], dp_grid[i][j+1])
+
+        # The original problem's answer is in dp_grid[0][0]. Return it.  
+        return dp_grid[0][0]
+
diff --git a/src/my_project/interviews/amazon_high_frequency_23/round_4/lowest_common_ancestor_binary_tree_iv.py b/src/my_project/interviews/amazon_high_frequency_23/round_4/lowest_common_ancestor_binary_tree_iv.py
new file mode 100644
index 00000000..71d4191d
--- /dev/null
+++ b/src/my_project/interviews/amazon_high_frequency_23/round_4/lowest_common_ancestor_binary_tree_iv.py
@@ -0,0 +1,29 @@
+from typing import List, Union, Collection, Mapping, Optional
+
+# Definition for a binary tree node.
+class TreeNode:
+    def __init__(self, x):
+        self.val = x
+        self.left = None
+        self.right = None
+
+class Solution:
+    def lowestCommonAncestor(self, root: TreeNode, nodes: List[TreeNode]) -> 'TreeNode':
+
+        node_set = set(nodes)
+
+        def dfs(node: TreeNode):
+
+            if not node or node in node_set:
+                return node 
+            
+            left = dfs(node.left)
+            right = dfs(node.right)
+
+            if left and right:
+                return node
+            
+            return left if left else right
+        
+        return dfs(root)
+        
\ No newline at end of file
diff --git a/src/my_project/interviews/top_150_questions_round_21/single_number.py b/src/my_project/interviews/top_150_questions_round_21/single_number.py
new file mode 100644
index 00000000..6751e0d3
--- /dev/null
+++ b/src/my_project/interviews/top_150_questions_round_21/single_number.py
@@ -0,0 +1,12 @@
+from typing import List, Union, Collection, Mapping, Optional
+from abc import ABC, abstractmethod
+
+class Solution:
+    def singleNumber(self, nums: List[int]) -> int:
+
+        answer = 0
+
+        for num in nums:
+            answer ^= num
+
+        return answer
\ No newline at end of file
diff --git a/tests/test_150_questions_round_21/test_single_number_round_21.py b/tests/test_150_questions_round_21/test_single_number_round_21.py
new file mode 100644
index 00000000..2e682b41
--- /dev/null
+++ b/tests/test_150_questions_round_21/test_single_number_round_21.py
@@ -0,0 +1,11 @@
+import unittest
+from src.my_project.interviews.top_150_questions_round_21\
+.single_number import Solution
+
+class SingleNumberTestCase(unittest.TestCase):
+
+    def test_single_number(self):
+        solution = Solution()
+        output = solution.singleNumber(nums=[2,2,1])
+        target = 1 
+        self.assertEqual(output, target)
\ No newline at end of file