diff --git a/189_RotateArray.py b/189_RotateArray.py
new file mode 100644
index 00000000..ae04d8e8
--- /dev/null
+++ b/189_RotateArray.py
@@ -0,0 +1,23 @@
+"""
+TC: O(n)
+SC: O(1)
+3 reverse operations
+reverse(0, n-k-1)
+reverse(n-k,n-1)
+reverse(0,n-1)
+"""
+class Solution:
+    def rotate(self, nums: List[int], k: int) -> None:
+        """
+        Do not return anything, modify nums in-place instead.
+        """
+        def reverse(start, end, nums):
+            while(start<end):
+                nums[start], nums[end] = nums[end], nums[start]
+                start+=1
+                end-=1
+        n = len(nums)
+        k = k%n
+        reverse(0,n-k-1,nums)
+        reverse(n-k,n-1,nums)
+        reverse(0,n-1,nums)
\ No newline at end of file
diff --git a/274_HIndex2.py b/274_HIndex2.py
new file mode 100644
index 00000000..82442e6b
--- /dev/null
+++ b/274_HIndex2.py
@@ -0,0 +1,20 @@
+"""
+TC: O(nlogn)+O(log n) ~ O(nlogn)
+SC: O(1)
+Sort and binary search
+"""
+class Solution:
+    def hIndex(self, citations: List[int]) -> int:
+        citations.sort()
+        n = len(citations)
+        l = 0
+        h = len(citations)-1
+        while(l<=h):
+            m = l+(h-l)//2
+            if citations[m]==n-m:
+                return citations[m]
+            elif citations[m]>n-m:
+                h=m-1
+            else:
+                l = m+1
+        return n-l
\ No newline at end of file
diff --git a/42_TrappingRainWater.py b/42_TrappingRainWater.py
new file mode 100644
index 00000000..5f090f73
--- /dev/null
+++ b/42_TrappingRainWater.py
@@ -0,0 +1,28 @@
+"""
+TC: O(n)
+SC: O(1)
+Logic: 2 pointer method. Maintain a left and right max
+"""
+class Solution:
+    def trap(self, height: List[int]) -> int:
+        leftMax = rightMax = 0
+        l = 0
+        r = len(height)-1
+        total_water = 0
+        while(l<r):
+            #process smaller height first
+            if height[l]<height[r]:
+                #process left side
+                if height[l]>leftMax:
+                    leftMax = height[l]
+                else:
+                    total_water+= leftMax-height[l]
+                l+=1
+            else:
+                #process right side
+                if height[r]>rightMax:
+                    rightMax = height[r]
+                else:
+                    total_water+= rightMax - height[r]
+                r-=1
+        return total_water
\ No newline at end of file