diff --git a/src/my_project/interviews/amazon_high_frequency_23/common_algos/two_sum_round_4.py b/src/my_project/interviews/amazon_high_frequency_23/common_algos/two_sum_round_4.py
new file mode 100644
index 00000000..6c0c6330
--- /dev/null
+++ b/src/my_project/interviews/amazon_high_frequency_23/common_algos/two_sum_round_4.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_4.py b/src/my_project/interviews/amazon_high_frequency_23/common_algos/valid_palindrome_round_4.py
new file mode 100644
index 00000000..e6f69dfb
--- /dev/null
+++ b/src/my_project/interviews/amazon_high_frequency_23/common_algos/valid_palindrome_round_4.py
@@ -0,0 +1,22 @@
+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 a 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/kids_with_the_greates_number_of_candies.py b/src/my_project/interviews/amazon_high_frequency_23/round_3/kids_with_the_greates_number_of_candies.py
new file mode 100644
index 00000000..b9272008
--- /dev/null
+++ b/src/my_project/interviews/amazon_high_frequency_23/round_3/kids_with_the_greates_number_of_candies.py
@@ -0,0 +1,8 @@
+from typing import List, Union, Collection, Mapping, Optional
+from abc import ABC, abstractmethod
+
+class Solution:
+    def kidsWithCandies(self, candies: List[int], extraCandies: int) -> List[bool]:
+        max_candies = max(candies)
+        return [candy + extraCandies >= max_candies for candy in  candies]
+
diff --git a/src/my_project/interviews/amazon_high_frequency_23/round_3/longest_common_subsequence.py b/src/my_project/interviews/amazon_high_frequency_23/round_3/longest_common_subsequence.py
new file mode 100644
index 00000000..39e0ec5f
--- /dev/null
+++ b/src/my_project/interviews/amazon_high_frequency_23/round_3/longest_common_subsequence.py
@@ -0,0 +1,23 @@
+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/top_150_questions_round_21/summary_ranges.py b/src/my_project/interviews/top_150_questions_round_21/summary_ranges.py
new file mode 100644
index 00000000..931f744a
--- /dev/null
+++ b/src/my_project/interviews/top_150_questions_round_21/summary_ranges.py
@@ -0,0 +1,30 @@
+from typing import List, Union, Collection, Mapping, Optional
+from abc import ABC, abstractmethod
+
+class Solution:
+    def summaryRanges(self, nums: List[int]) -> List[str]:
+
+        len_nums = len(nums)
+
+        if len_nums == 0:
+            return []
+        elif len_nums == 1:
+            return [f'{nums[0]}']
+        else:
+
+            answer = list()
+
+            pre = start = nums[0]
+
+            for i in nums[1:]:
+
+                if i - pre > 1:
+                    answer.append(f'{start}->{pre}' if pre-start > 0 else 
+                                  f'{start}')
+                    start = i
+
+                pre = i
+
+            answer.append(f'{start}->{pre}' if pre-start > 0 else f'{start}')
+
+            return answer
\ No newline at end of file
diff --git a/tests/test_150_questions_round_21/test_summary_ranges_round_21.py b/tests/test_150_questions_round_21/test_summary_ranges_round_21.py
new file mode 100644
index 00000000..e8200a42
--- /dev/null
+++ b/tests/test_150_questions_round_21/test_summary_ranges_round_21.py
@@ -0,0 +1,24 @@
+import unittest
+from src.my_project.interviews.top_150_questions_round_21\
+.summary_ranges import Solution
+
+class SummaryRangesTestCase(unittest.TestCase):
+
+    def test_empty(self):
+        solution = Solution()
+        output = solution.summaryRanges(nums=[])
+        target = []
+        self.assertEqual(target, output)
+
+    def test_single_element(self):
+        solution = Solution()
+        output = solution.summaryRanges(nums=[1])
+        target = ['1']
+        self.assertEqual(target, output)
+
+    def test_several_elements(self):
+        solution = Solution()
+        output = solution.summaryRanges(nums=[0,1,2,4,5,7])
+        target = ["0->2","4->5","7"]
+        for k, v in enumerate(target):
+            self.assertEqual(v, output[k])