class Solution {
public boolean isIsomorphic(String s, String t) {
if(s.length() != t.length()) return false;
HashMap<Character, Character> st = new HashMap<>();
HashMap<Character, Character> ts = new HashMap<>();
for(int i = 0; i < s.length(); i ++){
char a = s.charAt(i);
char b = t.charAt(i);
if(st.containsKey(a) && st.get(a) != b) return false;
st.put(a, b);
if(ts.containsKey(b) && ts.get(b) != a) return false;
ts.put(b, a);
}
return true;
}
}