Now update count array so that count[d] is the index in the array where the first number with digit d should go in sorted order for the position for this pass.

Just update count like this starting with d = 0:

 count[d] = count[d] + count[d - 1]
   

                       old    updated
      a               count   count
 0   412           0    0       0
 1   012           1    1       1
 2   101           2    2       3
 3   432           3    4       7
 4   302           4    0       7
 5   201           5    0       7
 6   410           6    0       7
                   .    .       .
                   .    .       .
                   9    0       7
   

Note that index 7 is out of bounds. But for this pass the position is the least significant digit (position 0) and only the digits 0, 1, and 2 occur.