diff --git a/best-time-to-buy-and-sell-stock/saysimple.py b/best-time-to-buy-and-sell-stock/saysimple.py
new file mode 100644
index 000000000..f64093fc7
--- /dev/null
+++ b/best-time-to-buy-and-sell-stock/saysimple.py
@@ -0,0 +1,18 @@
+"""
+https://leetcode.com/problems/best-time-to-buy-and-sell-stock/
+"""
+# - time complexity : O(n)
+# - space complexity : O(n)
+
+class Solution:
+    def maxProfit(self, prices: List[int]) -> int:
+        now = prices[0]
+        diff = [0] * len(prices)
+
+        for i, v in enumerate(prices[1:]):
+            if now > v:
+                now = v
+            else:
+                diff[i+1] = v - now
+
+        return max(diff)
diff --git a/contains-duplicate/saysimple.py b/contains-duplicate/saysimple.py
new file mode 100644
index 000000000..f7d206534
--- /dev/null
+++ b/contains-duplicate/saysimple.py
@@ -0,0 +1,14 @@
+"""
+https://leetcode.com/problems/contains-duplicate/
+"""
+# - time complexity : O(n)
+# - space complexity : O(n)
+
+class Solution:
+    def containsDuplicate(self, nums: List[int]) -> bool:
+        visited = {}
+        for n in nums:
+            if n in visited:
+                return True
+            visited[n] = True
+        return False
diff --git a/two-sum/saysimple.py b/two-sum/saysimple.py
new file mode 100644
index 000000000..5e8e101e9
--- /dev/null
+++ b/two-sum/saysimple.py
@@ -0,0 +1,21 @@
+"""
+https://leetcode.com/problems/two-sum/
+"""
+# - time complexity : O(nlogn)
+# - space complexity : O(n)
+
+class Solution:
+    def twoSum(self, nums: List[int], target: int) -> List[int]:
+        s, e = 0, len(nums) - 1
+        arr = [nums[i]:=[n,i] for i, n in enumerate(nums)]
+
+        arr.sort(key=lambda x: x[0])
+
+        while s <= e:
+            now = arr[s][0] + arr[e][0]
+            if now == target:
+                return [arr[s][1], arr[e][1]]
+            if now < target:
+                s += 1
+            else:
+                e -= 1
diff --git a/valid-anagram/saysimple.py b/valid-anagram/saysimple.py
new file mode 100644
index 000000000..c733af351
--- /dev/null
+++ b/valid-anagram/saysimple.py
@@ -0,0 +1,11 @@
+"""
+https://leetcode.com/problems/valid-anagram/
+"""
+# - time complexity : O(n)
+# - space complexity : O(n)
+
+from collections import Counter
+
+class Solution:
+    def isAnagram(self, s: str, t: str) -> bool:
+        return Counter(s) == Counter(t)
diff --git a/valid-palindrome/saysimple.py b/valid-palindrome/saysimple.py
new file mode 100644
index 000000000..f95dbafb1
--- /dev/null
+++ b/valid-palindrome/saysimple.py
@@ -0,0 +1,10 @@
+"""
+https://leetcode.com/problems/valid-palindrome/
+"""
+# - time complexity : O(n)
+# - space complexity : O(n)
+
+class Solution:
+    def isPalindrome(self, s: str) -> bool:
+        s = "".join([i for i in s if i.isalnum()]).lower()
+        return s == s[::-1]