Is Somorphic高频题需要bug free

public boolean isSomorphic(String s,String t){
if(s==null||t==null){
return false;
}
if(s.length()!=t.length()){
return false;
}
HashMap<Character,Character> map=new HashMap<Character,Character>();
for(int i=0;i<s.length();i++){
char c1=s.charAt(i);
char c2=t.charAt(i);
if(map.containsKey(c1)){
if(map.get(c1)!=c2){
return false;
}
}else{
if(map.containsValue(c2)){
return false;
}
map.put(c1, c2);
}
}
return true;
}