class SparseMatrix:
    def __init__(self):
        self.__matrix = dict()

    def save(self, matrix) -> None:
        for row, values in enumerate(matrix):
            for col, value in enumerate(values):
                if value != 0:
                    self.__matrix[(row, col)] = value

    def get(self, row, col) -> int or None:
        if self.__matrix[(row, col)]:
            return self.__matrix[(row, col)]

    def __str__(self) -> str:
        return '\n'.join(map(lambda pair: ' : '.join(map(str, pair)), self.__matrix.items()))


if __name__ == "__main__":
    # 5.1
    sparse_matrix = SparseMatrix()
    sparse_matrix.save([[3, 0, -2, 11], [0, 0, 9, 0], [0, 7, 0, 0], [0, 0, 0, 0]])
    print(sparse_matrix)
    print(sparse_matrix.get(0, 0))