Optimized Trapping Rainwater Solution to O(1) Space (Two-Pointer Approach)

dynamic_programming/trapped_water.py

@@ -34,22 +34,23 @@ def trapped_rainwater(heights: tuple[int, ...]) -> int:
         return 0
     if any(h < 0 for h in heights):
         raise ValueError("No height can be negative")
-    length = len(heights)
-
-    left_max = [0] * length
-    left_max[0] = heights[0]
-    for i, height in enumerate(heights[1:], start=1):
-        left_max[i] = max(height, left_max[i - 1])
-
-    right_max = [0] * length
-    right_max[-1] = heights[-1]
-    for i in range(length - 2, -1, -1):
-        right_max[i] = max(heights[i], right_max[i + 1])
-
-    return sum(
-        min(left, right) - height
-        for left, right, height in zip(left_max, right_max, heights)
-    )
+    left, right = 0, len(heights) - 1
+    leftmax, rightmax = 0, 0
+    water = 0
+    while left < right:
+        if heights[left] < heights[right]:
+            if heights[left] >= leftmax:
+                leftmax = heights[left]
+            else:
+                water += leftmax - heights[left]
+            left += 1
+        else:
+            if heights[right] >= rightmax:
+                rightmax = heights[right]
+            else:
+                water += rightmax - heights[right]
+            right -= 1
+    return water
 
 
 if __name__ == "__main__":