':type H: int
:type W: str'

def __init__(self, H, W):

    self.__exl = [[0 for _ in xrange(((ord(W) - ord('A')) + 1))] for _ in xrange((H + 1))]
    self.__fward = collections.defaultdict((lambda : collections.defaultdict(int)))
    self.__bward = collections.defaultdict(set)
