Frame Stacking#3220
Consider the following picture frames placed on an 9 \times 8 array.
Now place them on top of one another starting with at the bottom and ending up with on top. If any part of a frame covers another it hides that part of the frame below.
Viewing the stack of frames we see the following.
In what order are the frames stacked from bottom to top? The answer is EDABC.
Your problem is to determine the order in which the frames are stacked from bottom to top given a picture of the stacked frames. Here are the rules:
The width of the frame is always exactly character and the sides are never shorter than characters.
It is possible to see at least one part of each of the four sides of a frame. A corner shows two sides.
The frames will be lettered with capital letters, and no two frames will be assigned the same letter.
Each input block contains the height, (h \le 30) on the first line and the width (w \le 30) on the second. A picture of the stacked frames is then given as strings with characters each.
Write the solution to the standard output. Give the letters of the frames in the order they were stacked from bottom to top. If there are multiple possibilities for an ordering, list all such possibilities in alphabetical order, each one on a separate line. There will always be at least one legal ordering for each input block. List the output for all blocks in the input sequentially, without any blank lines (not even between blocks).
Example 1