1. , where is the initial number of empty rows.

2. In each row and in each column, the heights are all different.

3. A number in the left column counts how many tower tops are visible, looking horizontally from the left.

4. Similarly, the same as 3 is true looking from the right column, looking down from the top row, or looking up from the bottom row.

Copy the puzzle and print it, fill in the blank spaces and then check your solution.