Given a CSR Matrix, convert it to a Dense Matrix without using scipy.sparse or other pre-built functions. It must be done from scratc in Python 3.

You are given A, the matrix A in CSR format. A_row, the row start pointers, A_col.indices, the column indices for the non-zero values, and A_zero, the non zero values in A. You want to output B, a dense, 2D numpy array.