Write a function that returns True if the binary string can be rearranged to form a string of alternating 0s and 1s.
can_alternate("0001111") ➞ True
# Can make: "1010101"
can_alternate("01001") ➞ True
# Can make: "01010"
can_alternate("010001") ➞ False
can_alternate("1111") ➞ False
00 or 11 are not allowed).False if a string only contains 0s or only contains 1s.