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.

核心思想就是只要单词的每个字母是”一一映射“，就符合条件，每次如果没有映射关系，就建立单词a到单词b的相应位置的字母映射，如果已经存在这样的映射，就检查是否符合对应关系，即key value是否对应

public boolean isIsomorphic(String s, String t)
	{
		int slen=s.length();
		int tlen=t.length();
		if(slen!=tlen)
			return false;
		HashMap<Character, Character> hashmap=new HashMap<>();
		for(int i=0;i<slen;i++)
		{
			char schar=s.charAt(i);
			char tchar=t.charAt(i);
			if(!hashmap.containsKey(schar)&&!hashmap.containsValue(tchar))
				hashmap.put(schar, tchar);
			else {
				if(hashmap.get(schar)==null||hashmap.get(schar)!=tchar)
					return false;
			}
		}
		return true;
	}