String(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.

	 public static boolean isIsomorphic(String s, String t) {
	      if(s==null && t==null)  return true;
	      else if(s.length()!=t.length())  return false;
	      Map<Character,Character> hm=new HashMap<Character,Character>();
	      for(int i=0;i<s.length();i++) {
	    	  if(hm.containsKey(s.charAt(i))) {
	    		  if(hm.get(s.charAt(i))!=t.charAt(i))
	    			  return false;
	    	  }
	    	  else if(hm.containsValue(t.charAt(i)))
	    		  return false;   //开始的时候忘了这个条件，要注意一下
	    	  else
	    		  hm.put(s.charAt(i), t.charAt(i));
	      }
	      return true;
	    }