diff --git a/src/my_project/interviews/amazon_high_frequency_23/common_algos/two_sum_round_8.py b/src/my_project/interviews/amazon_high_frequency_23/common_algos/two_sum_round_8.py
new file mode 100644
index 00000000..6c0c6330
--- /dev/null
+++ b/src/my_project/interviews/amazon_high_frequency_23/common_algos/two_sum_round_8.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_8.py b/src/my_project/interviews/amazon_high_frequency_23/common_algos/valid_palindrome_round_8.py
new file mode 100644
index 00000000..1ed43720
--- /dev/null
+++ b/src/my_project/interviews/amazon_high_frequency_23/common_algos/valid_palindrome_round_8.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 lowercase
+        s = s.lower()
+
+        # Remove non-alphanumeric characters
+        s = re.sub(pattern='[^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_3/number_of_islands.py b/src/my_project/interviews/amazon_high_frequency_23/round_3/number_of_islands.py
new file mode 100644
index 00000000..cc03860f
--- /dev/null
+++ b/src/my_project/interviews/amazon_high_frequency_23/round_3/number_of_islands.py
@@ -0,0 +1,67 @@
+from typing import List, Union, Collection, Mapping, Optional
+from collections import deque
+
+class Solution:
+    def numIslands(self, grid: List[List[str]]) -> int:
+        """BFS approach - explores island level by level"""
+
+        if not grid:
+            return 0
+        
+        rows, cols = len(grid), len(grid[0])
+        islands = 0
+
+        def dfs(r: int, c: int):
+            queue = deque([(r,c)])
+            grid[r][c] = 0
+
+            while queue:
+                row, col = queue.popleft()
+
+                for dr, dc in [(-1,0), (1,0), (0,-1), (0,1)]:
+                    nr, nc = row + dr, col + dc
+                    if (0 <= nr < rows 
+                        and 0 <= nc < cols 
+                        and grid[nr][nc] == '1'):
+                        grid[nr][nc] = '0'
+                        queue.append((nr,nc))
+
+        for r in range(rows):
+            for c in range(cols):
+                if grid[r][c] == '1':
+                    islands += 1
+                    dfs(r,c)
+
+        return islands
+    
+
+    def numIslands_DFS(grid: List[List[str]]) -> int:
+        """DFS approach - explores island depth-first"""
+
+        if not grid:
+            return 0
+
+        rows, cols = len(grid), len(grid[0])
+
+        islands = 0
+
+        def dfs(r: int, c: int):
+            if (r < 0 or r >= rows or c < 0 or c >= cols or grid[r][c] == '0'):
+                return 
+            
+            grid[r][c] = '0'
+
+            dfs(r-1, c)
+            dfs(r+1, c)
+            dfs(r,c-1)
+            dfs(r,c+1)
+
+        for r in range(rows):
+            for c in range(cols):
+                if grid[r][c] == '1':
+                    islands += 1
+                    dfs(r,c)
+
+        return islands
+
+        
diff --git a/src/my_project/interviews/amazon_high_frequency_23/round_3/number_of_ways_to_select_building.py b/src/my_project/interviews/amazon_high_frequency_23/round_3/number_of_ways_to_select_building.py
new file mode 100644
index 00000000..dccb5781
--- /dev/null
+++ b/src/my_project/interviews/amazon_high_frequency_23/round_3/number_of_ways_to_select_building.py
@@ -0,0 +1,44 @@
+from typing import List, Union, Collection, Mapping, Optional
+from abc import ABC, abstractmethod
+
+class Solution:
+    def numberOfWays(self, s: str) -> int:
+        """
+        Count valid 3-building selections forming "010" or "101" patterns.
+        
+        Key insight: Track how many 2-character patterns we can form,
+        then extend them with the appropriate third character.
+        
+        Time: O(n), Space: O(1)
+        """
+
+        count_0 = 0  # Count of '0' seen so far
+        count_1 = 0  # Count of '1' seen so far
+        count_01 = 0  # Count of "01" patterns (0 followed by 1)
+        count_10 = 0  # Count of "10" patterns (1 followed by 0)
+        
+        result = 0
+        
+        for char in s:
+            if char == '0':
+                # Current '0' can complete "010": we need "01" before this '0'
+                result += count_01
+                
+                # Current '0' can start new "01" patterns: pair with each previous '0'
+                # Wait no, "01" means 0 then 1, so we can't create "01" with another 0
+                
+                # Current '0' extends all previous '1' to form "10" pattern
+                count_10 += count_1
+                
+                count_0 += 1
+            else:  # char == '1'
+                # Current '1' can complete "101": we need "10" before this '1'
+                result += count_10
+                
+                # Current '1' extends all previous '0' to form "01" pattern
+                count_01 += count_0
+                
+                count_1 += 1
+        
+        return result
+
diff --git a/src/my_project/interviews/top_150_questions_round_21/invert_binary_tree.py b/src/my_project/interviews/top_150_questions_round_21/invert_binary_tree.py
new file mode 100644
index 00000000..70eddd87
--- /dev/null
+++ b/src/my_project/interviews/top_150_questions_round_21/invert_binary_tree.py
@@ -0,0 +1,21 @@
+from typing import List, Union, Collection, Mapping, Optional
+from abc import ABC, abstractmethod
+
+class TreeNode:
+    def __init__(self, val=0, left=None, right=None):
+        self.val = val
+        self.left = left
+        self.right = right
+
+class Solution:
+    def invertTree(self, root: TreeNode) -> TreeNode:
+
+        try:
+            root.val
+        except:
+            return root
+        
+        root.left, root.right = (self.invertTree(root.right),
+                                 self.invertTree(root.left))
+        
+        return root
\ No newline at end of file
diff --git a/tests/test_150_questions_round_21/test_invert_binary_tree_round_21.py b/tests/test_150_questions_round_21/test_invert_binary_tree_round_21.py
new file mode 100644
index 00000000..74f1f98a
--- /dev/null
+++ b/tests/test_150_questions_round_21/test_invert_binary_tree_round_21.py
@@ -0,0 +1,19 @@
+import unittest
+from src.my_project.interviews.top_150_questions_round_21\
+.invert_binary_tree import Solution, TreeNode
+
+class InvertTreeTestCase(unittest.TestCase):
+
+    def test_none_inverted_tree(self):
+        solution = Solution()
+        tree = None
+        output = solution.invertTree(root=tree)
+        self.assertIsNone(output)
+
+    def test_inverted_tree(self):
+        solution = Solution()
+        tree = TreeNode(1,TreeNode(2),TreeNode(3))
+        output = solution.invertTree(root=tree)
+        self.assertEqual(1,output.val)
+        self.assertEqual(2,output.right.val)
+        self.assertEqual(3,output.left.val)
\ No newline at end of file