Isomorphic String

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.

不仅要判断s对应的t字符相同，还要判断t对应的s的字符相同。

如果用map存放s，t的字符，新字符在map中不仅key对应value，而且value对于key。

public boolean isIsomorphic(String s, String t) {if(s==null||t==null||s.length()!=t.length())return false;if(s==null&&t==null)return true;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);char c=getKey(map,c2);if(c!=' '&&c!=c1)return false;else if(map.containsKey(c1)){if(!map.get(c1).equals(c2))return false;}elsemap.put(c1,c2);}return true;}private char getKey(HashMap<Character,Character> map,char c){for(Map.Entry<Character, Character> entry:map.entrySet()){if(entry.getValue().equals(c))return entry.getKey();}return ' ';}