Add Binary.swift

extension String {

    var length: Int {
        return count
    }

    subscript (i: Int) -> String {
        return self[i ..< i + 1]
    }

    func substring(fromIndex: Int) -> String {
        return self[min(fromIndex, length) ..< length]
    }

    func substring(toIndex: Int) -> String {
        return self[0 ..< max(0, toIndex)]
    }

    subscript (r: Range<Int>) -> String {
        let range = Range(uncheckedBounds: (lower: max(0, min(length, r.lowerBound)),
                                            upper: min(length, max(0, r.upperBound))))
        let start = index(startIndex, offsetBy: range.lowerBound)
        let end = index(start, offsetBy: range.upperBound - range.lowerBound)
        return String(self[start ..< end])
    }
}

class Solution {
    func addBinary(_ a: String, _ b: String) -> String {
        
        let long = a.count >= b.count ? a : b
        let short = a.count < b.count ? a : b
        
        var sum: String = ""
        
        var j = short.count-1
        var carry1 = false
        
        for i in stride(from: long.count-1, through: 0, by: -1) {
            if j >= 0 {
//                print("\(long[i]) + \(short[j])")
                switch carry1 {
                case false:
                    if (long[i] == "0" && short[j] == "1") ||
                        (long[i] == "1" && short[j] == "0") {
                        sum = "1" + sum
                    } else if (long[i] == "0" && short[j] == "0") {
                        sum = "0" + sum
                    } else if (long[i] == "1" && short[j] == "1") {
                        sum = "0" + sum
                        carry1 = true
                    }
                case true:
                    if (long[i] == "0" && short[j] == "1") ||
                        (long[i] == "1" && short[j] == "0") {
                        sum = "0" + sum
                        // carry1 = true
                    } else if (long[i] == "0" && short[j] == "0") {
                        sum = "1" + sum
                        carry1 = false
                    } else if (long[i] == "1" && short[j] == "1") {
                        sum = "1" + sum
                        // carry1 = true
                    }
                }
                j -= 1
            } else {
                // the short string has reached the end. only the long string is left
                switch carry1 {
                case false:
                    sum = long[i] + sum
                case true:
                    if long[i] == "1" {
                        sum = "0" + sum
                        // carry1 = true
                    } else if long[i] == "0" {
                        sum = "1" + sum
                        carry1 = false
                    }
                }
            }
        }
        
        if carry1 {
            sum = "1" + sum
        }
        
        return sum
    }
}