Given a matrix a, return a matrix b in which all the zeros have "bubbled" to the top. That is, any zeros in a given column should be moved to the top. The order of the remaining nonzero numbers in the column should be preserved.

Example 1

 Input  a  = [ 1 2 3
               0 4 5
               6 0 0 ]

 Output b is [ 0 0 0
               1 2 3
               6 4 5 ]

Example 2

 Input  a  = [ 1 0 5 0 6 0 7 ]'
 Output b is [ 0 0 0 1 5 6 7 ]'

Example 3

 Input  a  = [ 1 0
               1 1 ]
 Output b is [ 1 0 
               1 1 ]