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