update solution of #42 [faster, shorter, more intuitive]

Author: ab12gu

python/0042-trapping-rain-water.py

@@ -1,18 +1,16 @@
 class Solution:
     def trap(self, height: List[int]) -> int:
-        if not height:
-            return 0
 
-        l, r = 0, len(height) - 1
-        leftMax, rightMax = height[l], height[r]
-        res = 0
-        while l < r:
-            if leftMax < rightMax:
-                l += 1
-                leftMax = max(leftMax, height[l])
-                res += leftMax - height[l]
-            else:
-                r -= 1
-                rightMax = max(rightMax, height[r])
-                res += rightMax - height[r]
-        return res
+        c = height.index(max(height))
+    
+        vol = 0
+        for arr in [height[:c], height[:c:-1]]:
+            first = 0
+            for i in arr:
+                if i < first:
+                    vol += first - i
+                else:
+                    first = i
+    
+        return vol
+