OneAway.Swift

 func oneAway(s1: String, s2: String) -> Bool {
        guard abs(s1.count - s2.count) <= 1 else { return false }
        var foundDifference = false
        var index1 = 0
        var index2 = 0
        
        while index1 < s1.count && index2 < s2.count {
            let c1 = s1[s1.index(s1.startIndex, offsetBy: index1)]
            let c2 = s2[s2.index(s1.startIndex, offsetBy: index2)]
            if c1 != c2 {
                if foundDifference { return false }
                foundDifference = true
                if index1 != index2 {
                    return false
                }
                
                if s1.count > s2.count {
                    index1 += 1
                    continue
                } else if s2.count > s1.count {
                    index2 += 1
                    continue
                }
            }
            index1 += 1
            index2 += 1
        }
        
        return true
    }