diff --git a/challenge-1/two_sum.py b/challenge-1/two_sum.py
new file mode 100644
index 0000000..734c948
--- /dev/null
+++ b/challenge-1/two_sum.py
@@ -0,0 +1,13 @@
+class Two_sum:
+    def twosum(self,target,nums):
+        prev_map=dict()
+        for i,num in enumerate(nums):
+            if target - num in prev_map:
+                return[i,prev_map[target-num]]
+            prev_map[num] = i
+
+two_sum = Two_sum()
+print(two_sum.twosum(9,[2,7,11,15,9]))
+      
+   
+   
\ No newline at end of file
diff --git a/challenge-2/Best_time_to_buyandsell_stock.py b/challenge-2/Best_time_to_buyandsell_stock.py
new file mode 100644
index 0000000..e7dbe64
--- /dev/null
+++ b/challenge-2/Best_time_to_buyandsell_stock.py
@@ -0,0 +1,29 @@
+"""
+Best Time to Buy and Sell Stock 
+
+You are given an array prices where prices[i] is the price of a given stock on the ith day.
+
+You want to maximize your profit by choosing a single day to buy one stock and choosing a different day in the future to sell that stock.
+
+Return the maximum profit you can achieve from this transaction. If you cannot achieve any profit, return 0.
+"""
+
+
+class Solution:
+    def maxProfit(self, prices):
+        #Two-pointer
+        left,right=0,1
+        profit=0
+
+        while right < len(prices):
+            #profitable
+            if prices[right] > prices[left]:
+                profit=max(profit,prices[right] - prices[left])
+            else:
+            #nonprofitable
+                left=right
+            right += 1
+        return profit
+
+stock= Solution()
+print(stock.maxProfit([7,1,5,3,6,4]))
diff --git a/challenge-3/Contains_Duplicate.py b/challenge-3/Contains_Duplicate.py
new file mode 100644
index 0000000..636fed8
--- /dev/null
+++ b/challenge-3/Contains_Duplicate.py
@@ -0,0 +1,29 @@
+"""
+Given an integer array nums, return true if any value appears at least twice in the array, and return false if every element is distinct.
+Example 1:
+
+Input: nums = [1,2,3,1]
+
+Output: true
+
+Explanation:
+
+The element 1 occurs at the indices 0 and 3.
+"""
+
+class Solution:
+    def containsDuplicate(self, nums):
+        hash_set=set()
+        for num in nums:
+            if num in hash_set:
+                return True
+            else:
+                hash_set.add(num)
+        return False
+
+duplicate = Solution()
+print(duplicate.containsDuplicate([1,2,3,1]))
+print(duplicate.containsDuplicate([1,2,3,4]))
+
+
+                
diff --git a/challenge-4/Sum_of_Two_Integers.py b/challenge-4/Sum_of_Two_Integers.py
new file mode 100644
index 0000000..cc8b750
--- /dev/null
+++ b/challenge-4/Sum_of_Two_Integers.py
@@ -0,0 +1,18 @@
+# Given two integers a and b, return the sum of the two integers without using the operators + and -.
+
+# Example 1:
+# Input: a = 1, b = 2
+# Output: 3
+
+class Solution:
+    def getSum(self, a, b) :
+        bitshortner = 0xFFFFFFFF
+
+        while (b & bitshortner) > 0:
+            carry = (a & b) << 1
+            a = a ^ b
+            b = carry
+        return (a & bitshortner) if b > 0 else a
+
+sum = Solution()
+print(sum.getSum(1,2))
diff --git a/challenge-5/Product_of_Array_Except_Self.py b/challenge-5/Product_of_Array_Except_Self.py
new file mode 100644
index 0000000..a1f403f
--- /dev/null
+++ b/challenge-5/Product_of_Array_Except_Self.py
@@ -0,0 +1,21 @@
+# https://leetcode.com/problems/product-of-array-except-self/
+
+# Product of Array Except Self
+
+
+class Solution:
+    def productExceptSelf(self, nums):
+        res=[1] * (len(nums))
+        prefix=1
+        for i in range(len(nums)):
+            res[i] = prefix
+            prefix *= nums[i]
+        postfix=1
+        for i in range(len(nums)-1, -1, -1):
+            res[i] *= postfix
+            postfix *= nums[i]
+        return res
+
+
+product = Solution()
+print(product.productExceptSelf([1,2,3,4]))