Tetris Alphabet#2646
The game of Tetris is played with four-connected blocks falling down a well with rows and columns. Each figure is marked with a unique English letter from to .
Your program must determine the order in which the blocks fell, given the state of the well.
The first line contains the integer (1 \leq N \leq 50) — the number of rows. The next lines contain characters each, where each character is either a letter from to or a dot character (), representing an empty cell.
Output a string of letters indicating the order in which the figures fell. If more than one order is possible, output the lexicographically smallest one. The input is guaranteed to have at least one nonempty order.
Example 1