class Solution {
    func isIsomorphic(_ s: String, _ t: String) -> Bool {

        let sChars = Array(s)
        let tChars = Array(t)

        var mappingCharCodes = [Character: Character]()
        var backMappingCharCodes = [Character: Character]()

        if s.count == 0 { return true }

        for index in 0 ... (s.count - 1) {

            let sChar = sChars[index]
            let tChar = tChars[index]

            if let mappingTChar = mappingCharCodes[sChar] {
                if tChar != mappingTChar {
                    return false
                }
            }else if let mappingSChard = backMappingCharCodes[tChar] {
                if sChar != mappingSChard {
                    return false
                }
            }else {
                mappingCharCodes.updateValue(tChar, forKey: sChar)
                backMappingCharCodes.updateValue(sChar, forKey: tChar)
            }
        }
        return true
    }
}