67. Add Binary

    func addBinary(_ a: String, _ b: String) -> String {
            let arr1 = Array(a)
            let arr2 = Array(b)
            var res = ""
            var p1 = arr1.count - 1
            var p2 = arr2.count - 1
            var carry = 0
            while p1 >= 0 || p2 >= 0 || carry > 0{
                let n1 = p1 >= 0 ? Int(String(arr1[p1]))! : 0
                let n2 = p2 >= 0 ? Int(String(arr2[p2]))! : 0
                let sum = n1 + n2 + carry
                carry = sum / 2
                res = String(sum % 2) + res
                p1 -= 1
                p2 -= 1
            }
        return res
    }