矩阵的稀疏存储方式

DOK：Dictionary Of Keys

• https://docs.scipy.org/doc/scipy/reference/generated/scipy.sparse.dok_matrix.html
• O(1) access of individual elements

COO：COOrdinate

• https://docs.scipy.org/doc/scipy/reference/generated/scipy.sparse.coo_matrix.html
• fast conversion among sparse formats
• permits duplicate entries (see example)

LIL：Row-based list of lists

• https://docs.scipy.org/doc/scipy/reference/generated/scipy.sparse.lil_matrix.html