In merge sort, at each level n decrease by a factor of 2,
Meaning, at the top level there are n elements,
it is decreasing as :
n/2,n/4,n/8,n/16..... upto 1
at levels 2,3,4,5...... upto log(n)
we stop when n=1 or a constant.
so how many steps does it take for n to decrease from n to 1 is log(n). If we are decreasing by a factor of two.