diff --git a/coin-change/i-mprovising.py b/coin-change/i-mprovising.py
new file mode 100644
index 000000000..912f44c18
--- /dev/null
+++ b/coin-change/i-mprovising.py
@@ -0,0 +1,30 @@
+"""
+Time complexity O(len(coins) * amount)
+Space complexity O(amount)
+
+Dynamic programming
+"""
+
+class Solution:
+    def coinChange(self, coins: List[int], amount: int) -> int:
+        coins = sorted(coins)
+        
+        if amount == 0 :
+            return 0
+
+        dp = [0] + [-1 for _ in range(amount)]
+
+        for i in range(coins[0], amount+1):
+            tmp = []
+            for x in coins:
+                if i - x >= 0:
+                    if dp[i - x] != -1:
+                        tmp.append(dp[i - x] + 1)
+            if len(tmp) == 0:
+                dp[i] = -1
+            else:
+                dp[i] = min(tmp)
+
+        if dp[-1] == 0:
+            return -1
+        return dp[-1]
diff --git a/find-minimum-in-rotated-sorted-array/i-mprovising.py b/find-minimum-in-rotated-sorted-array/i-mprovising.py
new file mode 100644
index 000000000..18e850cb7
--- /dev/null
+++ b/find-minimum-in-rotated-sorted-array/i-mprovising.py
@@ -0,0 +1,23 @@
+"""
+Time complexity O(logn)
+Space compexity O(1)
+
+Binary search
+"""
+
+class Solution:
+
+    def findMin(self, nums: List[int]) -> int:
+        start = 1
+        end = len(nums) - 1
+        
+        while start <= end:
+            mid = (start + end) // 2
+            if nums[mid-1] > nums[mid]:
+                return nums[mid]
+            if nums[0] < nums[mid]:
+                start = mid + 1
+            else:
+                end = mid - 1 
+
+        return nums[0]
diff --git a/maximum-depth-of-binary-tree/i-mprovising.py b/maximum-depth-of-binary-tree/i-mprovising.py
new file mode 100644
index 000000000..0206b658e
--- /dev/null
+++ b/maximum-depth-of-binary-tree/i-mprovising.py
@@ -0,0 +1,24 @@
+"""
+Time complexity O(n)
+"""
+from collections import deque
+
+class Solution:
+    def maxDepth(self, root: Optional[TreeNode]) -> int:
+        if not root:
+            return 0
+            
+        max_depth = 0
+
+        # bfs
+        q = deque([[root, 1]])
+        while(q):
+            node, depth = q.pop()
+            if max_depth < depth:
+                max_depth = depth
+            if node.left:
+                q.append([node.left, depth+1])
+            if node.right:
+                q.append([node.right, depth+1])
+
+        return max_depth
diff --git a/merge-two-sorted-lists/i-mprovising.py b/merge-two-sorted-lists/i-mprovising.py
new file mode 100644
index 000000000..d898b7dc8
--- /dev/null
+++ b/merge-two-sorted-lists/i-mprovising.py
@@ -0,0 +1,35 @@
+"""
+Time complexity O(n+m)
+"""
+
+class Solution:
+    def mergeTwoLists(self, list1: Optional[ListNode], list2: Optional[ListNode]) -> Optional[ListNode]:
+        if not (list1 or list2):
+            return list1 or list2 # if empty list
+
+        nums = []
+        while(list1 or list2):
+            if not list1:
+                val = list2.val
+                list2 = list2.next
+            elif not list2:
+                val = list1.val
+                list1 = list1.next
+            else:
+                if list1.val <= list2.val:
+                    val = list1.val
+                    list1 = list1.next
+                else:
+                    val = list2.val
+                    list2 = list2.next
+            nums.append(val)
+        
+        head = ListNode(nums[0])
+        node = head
+        if len(nums) == 1:
+            return head
+        for n in nums[1:]:
+            tmp = ListNode(n)
+            node.next = tmp
+            node = tmp
+        return head