Isomorphic Strings

Given two strings s and t, determine if they are isomorphic.

Two strings are isomorphic if the characters in s can be replaced to get t.

All occurrences of a character must be replaced with another character while preserving the order of characters. No two characters may map to the same character but a character may map to itself.

For example,
Given "egg", "add", return true.

Given "foo", "bar", return false.

Given "paper", "title", return true.

Note:
You may assume both s and t have the same length.

public class Solution {
    public boolean isIsomorphic(String s, String t) {
        if(s == null || t == null || s.length() != t.length()) return false;
        int len = s.length();
        HashMap<Character, Character> sTot = new HashMap<Character, Character> ();
        HashMap<Character, Character> tTos = new HashMap<Character, Character> ();
        for(int i = 0; i < len; i ++){
            char sc = s.charAt(i);
            char tc = t.charAt(i);
            if(!sTot.containsKey(sc)){ 
                sTot.put(sc, tc);
            }else{
                if(sTot.get(sc) != tc) return false;
            }
            //take care case: ab & aa
            if(!tTos.containsKey(tc)){
                tTos.put(tc, sc);
            }else{
                if(tTos.get(tc) != sc) return false;
            }
        }
        return true;
    }
}