diff --git a/3sum/Leo.py b/3sum/Leo.py
new file mode 100644
index 000000000..6b952a5ef
--- /dev/null
+++ b/3sum/Leo.py
@@ -0,0 +1,26 @@
+class Solution:
+    def threeSum(self, nums: List[int]) -> List[List[int]]:
+
+        nums.sort()
+        res = []
+
+        for i, ch in enumerate(nums):
+            if i > 0 and nums[i] == nums[i - 1]: continue
+
+            l, r = i + 1, len(nums) - 1
+            while l < r:
+                threeSum = ch + nums[l] + nums[r]
+
+                if threeSum < 0:
+                    l += 1
+                elif threeSum > 0:
+                    r -= 1
+                else:
+                    res.append([ch, nums[l], nums[r]])
+                    l += 1
+                    while l < r and nums[l] == nums[l - 1]:
+                        l += 1
+
+        return res
+
+        ## TC: O(n^2), SC: O(1)
diff --git a/encode-and-decode-strings/Leo.py b/encode-and-decode-strings/Leo.py
new file mode 100644
index 000000000..61f295a35
--- /dev/null
+++ b/encode-and-decode-strings/Leo.py
@@ -0,0 +1,26 @@
+class Codec:
+    def encode(self, strs: List[str]) -> str:
+        """Encodes a list of strings to a single string.
+        """
+        res = ''
+
+        for s in strs:
+            res += str(len(s)) + '#' + s
+
+        return res
+
+    def decode(self, s: str) -> List[str]:
+        """Decodes a single string to a list of strings.
+        """
+        res = []
+
+        i = 0
+        while i < len(s):
+            a = s.find('#', i)
+            length = int(s[i:a])
+            res.append(s[a + 1:a + 1 + length])
+            i = a + 1 + length
+
+        return res
+
+        ## TC: O(n), SC:O(n),n denotes sum of all len(s)
diff --git a/longest-consecutive-sequence/Leo.py b/longest-consecutive-sequence/Leo.py
new file mode 100644
index 000000000..b5c43afff
--- /dev/null
+++ b/longest-consecutive-sequence/Leo.py
@@ -0,0 +1,18 @@
+class Solution:
+    def longestConsecutive(self, nums: List[int]) -> int:
+
+        seen = set(nums)
+        longest = 0
+
+        for n in seen:
+            if (n - 1) not in seen:
+                length = 1
+
+                while (n + length) in seen:
+                    length += 1
+
+                longest = max(length, longest)
+
+        return longest
+
+        ## TC: O(n), SC: O(n)
diff --git a/product-of-array-except-self/Leo.py b/product-of-array-except-self/Leo.py
new file mode 100644
index 000000000..5a692524d
--- /dev/null
+++ b/product-of-array-except-self/Leo.py
@@ -0,0 +1,16 @@
+class Solution:
+    def productExceptSelf(self, nums: List[int]) -> List[int]:
+        length = len(nums)
+        res = [1] * length
+        ltor = 1
+        rtol = 1
+
+        for i in range(length):
+            res[i] *= ltor
+            ltor = ltor * nums[i]
+            res[length - i - 1] *= rtol
+            rtol = rtol * nums[length - i - 1]
+
+        return res
+
+        ## TC: O(n), SC: O(1) 
diff --git a/top-k-frequent-elements/Leo.py b/top-k-frequent-elements/Leo.py
new file mode 100644
index 000000000..75c26468a
--- /dev/null
+++ b/top-k-frequent-elements/Leo.py
@@ -0,0 +1,17 @@
+class Solution:
+    def topKFrequent(self, nums: List[int], k: int) -> List[int]:
+
+        num_dict = collections.Counter(nums)
+        freq = num_dict.most_common()
+        ans = []
+
+        for key, val in freq:
+            if k == 0:
+                break
+
+            ans.append(key)
+            k -= 1
+
+        return ans
+
+        ## TC: O(n * logn), SC: O(n)