Bear and Species

Bearland can be represented as a square grid that consists of N rows and N columns. Two cells are called adjacent if they share a side. In the input, each cell is described by one character: - '.' is an empty cell. - 'B', 'G' or 'P' is a cell inhabited by bears of one species — brown, grizzly or polar bears respectively. - '?' is a cell inhabited by bears of one species but you don't know wh