func isIsomorphic(s string, t string) bool {
if len(s) != len(t) {
return false
}
sToT := map[byte]byte{}
tToS := map[byte]byte{}
for idx := range s {
x, y := s[idx], t[idx]
if (sToT[x] > 0 && sToT[x] != y) || (tToS[y] > 0 && tToS[y] != x) {
return false
}
sToT[x] = y
tToS[y] = x
}
return true
}