```
class Solution(object):
def isIsomorphic(self, s, t):
first = [-1]*256
second = [-1]*256
length = len(s)
for i in range(len(s)):
if first[ord(s[i])] != second[ord(t[i])]:
return False
first[ord(s[i])] = i
second[ord(t[i])] = i
return True
```