# odd-numbered rows are right-shifted by half a tile width
class Map(object):
    def __init__(self):
        self._arr = []
        self._list = []
        self._finalized = False

    def add(self, row, col, tile):
        self._extend(row, col)
        self._arr[row][col] = tile
        self._list.append(tile)
        self._finalized = False

    def get(self, row, col):
        if row < 0 or len(self._arr) <= row:
            return None
        if col < 0 or len(self._arr[row]) <= col:
            return None
        return self._arr[row][col]

    def _extend(self, row, col):
        while len(self._arr) <= row:
            self._arr.append([])
        while len(self._arr[row]) <= col:
            self._arr[row].append(None)

    def _finalize(self):
        if self._finalized:
            return
        self._neighbors = {}
        def check(_r, _c, tile):
            n = self.get(_r, _c)
            if n:
                self._neighbors[_c].append(n)
        for row, r in enumerate(self._arr):
            for col, tile in enumerate(r):
                if tile:
                    self._neighbors[tile] = []
                    offset = row & 1
                for i in range(2):
                    c = col - 1 + i + offset
                    check(row - 1, c)
                    check(row + 1, c)
                check(row, col - 1)
                check(row, col + 1)
        self._finalized = True

    def number(self):
        self._finalize()