LeetCode 205. Isomorphic Strings

public class Solution {
public boolean isIsomorphic(String s, String t) {
if (s.length() == 0) return true;
Map<Character, Character> map = new HashMap<Character, Character>();
for (int i = 0; i < s.length(); i++) {
char cs = s.charAt(i);
char ct = t.charAt(i);
if (map.containsKey(cs)) {
if (map.get(cs) != ct) return false;
} else if (map.containsValue(ct)) return false;
else map.put(cs, ct);
}
return true;
}
}