Before revision:  We give the first randomized algorithms for this problem running in time ( \text{nnz  + \text{nnz  + c^2) \cdot k/\varepsilon^{1.
After revision:  We give the first randomized algorithms for this problem running in time ( \text{nnz + \text{nnz (B) + c^2) \cdot k/\varepsilon^{1.