Theory of Computation

Consider a puzzle, PUZZLE, where you start with a n x n matrix with a random allocation of three characters, X0 s, Y 0 s, and Z 0 s, in the grid locations. The goal of the puzzle is to remove the characters, one at a time, so that each row contains only characters of one letter, such as only X0 s, and each column contains at least one character. Show that PUZZLE ? NP.