Alternating Pattern

Python 2, 62 bytes

lambda n:["%*s"%(i%2*2*n-n,"x"*min(i+1,n-i))for i in range(n)]

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Uses x and space.

The rows are computed like this:

"%-5s" % "x"      == "x    "
"%5s"  % "xx"     == "   xx"
"%-5s" % "xxx"    == "xxx  "
"%5s"  % "xx"     == "   xx"
"%-5s" % "x"      == "x    "

Using the %*s specifier to choose between n and -n.

Jelly, 9 bytes

>þµoṚUÐeY

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Explanation

>þ           Create a table of (x>y) over [1…n]×[1…n]:
               [0 1 1 1 1]
               [0 0 1 1 1]
               [0 0 0 1 1]
               [0 0 0 0 1]
               [0 0 0 0 0]
  µ          Take this array, and...
   oṚ        OR it with its reverse:
               [0 1 1 1 1]
               [0 0 1 1 1]
               [0 0 0 1 1]
               [0 0 1 1 1]
               [0 1 1 1 1]
    UÐe      Apply U (reverse) to even-indexed rows.
       Y     Join by newlines.

APL (Dyalog Classic), 18 bytes

⎕a[↑⊢∘⌽\(⊂>⊢⌊⌽)⍳⎕]

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outputs AB instead of *#

⎕ evaluated input n

⍳⎕ the vector 0 1 ... n-1

⊢⌊⌽ min (⌊) between themselves (⊢) and their reverse (⌽) - see trains

⊂>⊢⌊⌽ where is the ⍳ vector as a whole (⊂) less than each of its ⊢⌊⌽ - return a vector of boolean (0/1) vectors

⊢∘⌽\ reverse every other vector

↑ mix into a matrix

⎕a the uppercase English alphabet, 'AB...Z'

⎕a[ ] replace 0 1 with 'A' 'B'