Skyscrapers 6x6 - Stuck at the end

I tried to write a program to solve a Skyscrapers puzzle. Until the last steps I always used the opposite clues. But I can’t find a solution for the last cells. This is where I am:

    3 2 2 3 2 1

    = = = = = = 
4  |2| | |3|5|6|  1
    = = = = = = 
2  |1|6|3|2|4|5|  2
    = = = = = = 
2  |4| |6|5| | |  3
    = = = = = = 
1  |6|5|2|1|3|4|  3
    = = = = = = 
2  |5| | |6| | |  2
    = = = = = = 
3  |3|2|5|4|6|1|  2
    = = = = = = 

    3 4 2 2 1 5

How can I determine the last cells here? What are the steps?

I know the solution and know that my steps until here are correct