題目描述: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.
分析：判斷字串s能否通過字元替換得到字串t

解題思路：將字串s和字串t的對應關係用map來儲存，當發現該字元在map中已經存在，則將map中的值取出看與當前字串t的字元是否一樣。如果不存在則需要將s與t的對應字元存入map中。需要兩次比較過程！

public class Isomorphic_Strings {

    public
 
 boolean isIsomorphic(String s, String t) {
        return doCheck(s,t)&&doCheck(t,s);
    }

    public boolean doCheck(String s,String t){
        Map<Character,Character> map = new HashMap<Character,Character>();
        if(s.length()!=t.length())return false;
        for(int i=0
 
;i<s.length();i++){
            char sChar = s.charAt(i);
            char tChar = t.charAt(i);
            if(!map.containsKey(sChar)){
                map.put(sChar, tChar);
            }else{
                char cCompare = map.get(sChar);
                if(cCompare!=tChar){
                    return false;
                }
            }
        }
        return true;
    }

}