Rules: Draw a single closed loop following the grid lines without touching itself even diagonally. Fill the grid with digits 1 to 4, so that each digit appears exactly twice in each row/column. Each digit represents a building with the height of that digit itself.
Each row and column should have exactly four cells inside the loop, containing digits 1 to 4 exactly once.
Clues on the grey line represents the number of buildings INSIDE the loop that can be seen from the corresponding direction (outside buildings are ignored). Clues on the white line represents the number of buildings OUTSIDE the loop that can be seen from the corresponding direction (inside buildings are ignored).