diff --git a/LeetCode/hard/trap_42.py b/LeetCode/hard/trap_42.py
new file mode 100644
index 0000000..23e8785
--- /dev/null
+++ b/LeetCode/hard/trap_42.py
@@ -0,0 +1,23 @@
+from typing import List
+
+
+class Solution:
+    def trap(self, height: List[int]) -> int:
+        if not height:
+            return 0
+
+        left, right = 0, len(height) - 1
+        left_max, right_max = height[left], height[right]
+        res = 0
+
+        while left < right:
+            if left_max < right_max:
+                left += 1
+                left_max = max(left_max, height[left])
+                res += left_max - height[left]
+            else:
+                right -= 1
+                right_max = max(right_max, height[right])
+                res += right_max - height[right]
+
+        return res
diff --git a/tests/test_leecode_hard.py b/tests/test_leecode_hard.py
new file mode 100644
index 0000000..65a1940
--- /dev/null
+++ b/tests/test_leecode_hard.py
@@ -0,0 +1,23 @@
+import pytest
+
+
+@pytest.mark.parametrize(
+    "height, expected",
+    [
+        ([0, 1, 0, 2, 1, 0, 1, 3, 2, 1, 2, 1], 6),
+        ([4, 2, 0, 3, 2, 5], 9),
+        ([], 0),
+        ([1], 0),
+        ([1, 2], 0),
+        ([1, 2, 3, 4, 5], 0),  # increasing
+        ([5, 4, 3, 2, 1], 0),  # decreasing
+        ([3, 3, 3, 3], 0),  # flat
+        ([3, 0, 3], 3),
+        ([5, 0, 5], 5),
+        ([2, 0, 2], 2),
+    ],
+)
+def test_trap(height, expected):
+    from LeetCode.hard.trap_42 import Solution
+
+    assert Solution().trap(height) == expected