205. 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 != null && t == null) || s.length() != t.length()){
            return false;
        }
        HashMap<Character, Character> map1 = new HashMap<Character, Character>();
        HashMap<Character, Character> map2 = new HashMap<Character, Character>();
        for(int i=0; i<s.length(); i++){
            if(map1.containsKey(s.charAt(i))){
                if(map1.get(s.charAt(i)) == t.charAt(i)){
                    continue;
                }else{
                    return false;
                }
            }else if(map2.containsKey(t.charAt(i))){
                if(map2.get(t.charAt(i)) == s.charAt(i)){
                    continue;
                }else{
                    return false;
                }
            }else{
                map1.put(s.charAt(i), t.charAt(i));
                map2.put(t.charAt(i), s.charAt(i));
            }
        }
        return true;
    }
}

考慮到ab, aa的狀況，要用2個hashmap