205. Isomorphic Strings

class Solution {
    func isIsomorphic(_ s: String, _ t: String) -> Bool {
        var map = [Character: Character]()
        for i in s.indices {
            if let value = map[s[i]] {
                guard value == t[i] else {
                    return false
                }
            }
            else if map.values.contains(t[i]) {
                return false
            }
            else {
                map[s[i]] = t[i]
            }
        }
        return true
    }
}