I have tried to solve this question using topological sort. I am getting correct output for 3 out of the 4 test cases given in the sample test cases on kickstart website. I will be very thankful if someone can help me find the mistake in my code. I have been trying to find any mistake but couldn't

Approach — To get the correct ordering of letters I have made a string out of all columns starting from last row to first row. I am adding a letter of its type only once in a string. Now using all these strings I am building a graph pointing from bottom row to top row. Now I am just applying topological sort to get the ordered string as the answer.

Here is the link to my solution — http://p.ip.fi/IVy2 Here is the link for ideone (you can see that I am getting correct output for 3/4 cases) : https://ideone.com/OXNJcW

Thank you