diff --git a/contains-duplicate/Raft.py b/contains-duplicate/Raft.py
new file mode 100644
index 000000000..bd5f51287
--- /dev/null
+++ b/contains-duplicate/Raft.py
@@ -0,0 +1,12 @@
+class Solution:
+    def containsDuplicate(self, nums: List[int]) -> bool:
+        appeared = defaultdict(int)
+        for n in nums:
+            if appeared[n] > 0:
+                return True
+            appeared[n] += 1
+        
+        return False
+# T: O(n)
+# S: O(n)
+
diff --git a/kth-smallest-element-in-a-bst/Raft.py b/kth-smallest-element-in-a-bst/Raft.py
new file mode 100644
index 000000000..5b21826f2
--- /dev/null
+++ b/kth-smallest-element-in-a-bst/Raft.py
@@ -0,0 +1,16 @@
+class Solution:
+    def kthSmallest(self, root: Optional[TreeNode], k: int) -> int:
+        stack = []
+        
+        while True:
+            while root:
+                stack.append(root)
+                root = root.left
+            root = stack.pop()
+            k -= 1
+            if not k:
+                return root.val
+            root = root.right
+# T: O(n)
+# S: O(n)
+
diff --git a/number-of-1-bits/Raft.py b/number-of-1-bits/Raft.py
new file mode 100644
index 000000000..48f74fa8c
--- /dev/null
+++ b/number-of-1-bits/Raft.py
@@ -0,0 +1,10 @@
+class Solution:
+    def hammingWeight(self, n: int) -> int:
+        res = 0
+        while n:
+            res += n % 2
+            n = n >> 1
+        return res
+# T: logn
+# S: 1
+
diff --git a/palindromic-substrings/Raft.py b/palindromic-substrings/Raft.py
new file mode 100644
index 000000000..904f9075f
--- /dev/null
+++ b/palindromic-substrings/Raft.py
@@ -0,0 +1,18 @@
+class Solution:
+    def countSubstrings(self, s: str) -> int:
+        count = 0
+        for i in range(len(s)):
+            count += self.countPalindrome(s, i, i)
+            count += self.countPalindrome(s, i, i + 1)
+        return count
+    
+    def countPalindrome(self, s, l, r):
+        count = 0
+        while r < len(s) and l >= 0 and s[l] == s[r]:
+            count += 1
+            l -= 1
+            r += 1
+        return count
+# T: O(n^2)
+# S: O(n^2)
+
diff --git a/top-k-frequent-elements/raft.py b/top-k-frequent-elements/raft.py
new file mode 100644
index 000000000..a92f75ece
--- /dev/null
+++ b/top-k-frequent-elements/raft.py
@@ -0,0 +1,44 @@
+class Solution:
+    def topKFrequent(self, nums: List[int], k: int) -> List[int]:
+        # Frequency
+        freq = defaultdict(int)
+        for n in nums:
+            freq[n] += 1
+        
+        freq_list = []
+        for key, val in freq.items():
+            freq_list.append((-1 * val, key))
+        
+        heapq.heapify(freq_list)
+        
+        res = []
+        for _ in range(k):
+            res.append(heapq.heappop(freq_list)[1])
+        
+        return res
+
+    # T: nlogn
+    # S: n
+
+    def topKFrequent2(self, nums: List[int], k: int) -> List[int]:
+        res = []
+        # Frequency
+        count = defaultdict(int)
+        for n in nums:
+            count[n] += 1
+
+        # Convert to frequency
+        frequency = [[] for _ in range(len(nums) + 1)]
+        for key, freq in count.items():
+            frequency[freq].append(key)
+
+        # top K
+        for freq in range(len(nums), 0, -1):
+            for n in frequency[freq]:
+                res.append(n)
+
+                if len(res) == k:
+                    return res
+    # T: n
+    # S: n
+