# Time: O(N · H) where H is the maximum height
# Space: O(1)

class Solution:
    def trap(self, height: List[int]) -> int:
        maxh = 0
        
        for h in height:
            maxh = max(h, maxh)
        
        land_width = len(height)
        output = 0
        
        for i in range(maxh):
            row_output = 0
            trough_output = 0
            trough_start = -1
            
            for j in range(land_width):
                # A block is present
                if height[j] >= i + 1:
                    row_output += trough_output
                    trough_start = j
                    trough_output = 0
                elif trough_start >= 0:
                    trough_output += 1
        
            output += row_output
        
        return output