I'm trying to understand how external sorting works from the book "Data Structures and Algorithm Analysis" by Mark Allen Weiss.
The author illustrates an example where he tries to sort 81,94,11,96,12,35,17,99,28,58,41 with a memory which can hold upto M=3 numbers. He uses two input tapes Ta1,Ta2 and two output tapes Tb1,Tb2.
Ta1 81,94,11,96,12,35,17,99,28,58,41 Ta2 Tb1 Tb2
In the first step M numbers are read into memory then, sorted and written to Tb1 and Tb2 alternatively.
Ta1 Ta2 Tb1 11,81,94 17,28,99 15 Tb2 12,35,96 41,58,75
I understand how the algorithm works but, don't understand why it is important to write results alternatively. Couldn't this be achieved without alternating?