205. Isomorphic Strings

public class Solution {
    public boolean isIsomorphic(String s, String t) {
           if(s==null&&t==null)
        return true;
        if(s==null||t==null||s.length()!=t.length())
        return false;
        HashMap<Character,Character> res1=new HashMap<Character,Character>();
        HashMap<Character,Character> res2=new HashMap<Character,Character>();
        char[] sToArray=s.toCharArray();
        char[] tToArray=t.toCharArray();
     
        for(int i=0;i<sToArray.length;i++){
            if((!res1.containsKey(sToArray[i]))&&(!res2.containsKey(tToArray[i]))){
                res1.put(sToArray[i],tToArray[i]);
                res2.put(tToArray[i],sToArray[i]);
                
            }
            
            else { if(res1.get(sToArray[i])==null||res2.get(tToArray[i])==null)//你必须要有这个判断，否则下个if会出现nullpointerexception，
            //原因的话在overflow有人解释了 如果你Character和char比较，有个unopening box 过程，这个时候如果是空指针，就会出现这个exception
            return false;
              if(((res1.get(sToArray[i])!=tToArray[i])||(res2.get(tToArray[i])!=sToArray[i])))
             return false;
                
            }
            
        }
        return true;
        
    }
}