Isomorphic Strings

Given two strings s and t, determine if they are isomorphic. Two strings s and t 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.

Example 1:
Input: s = "egg", t = "add"
Output: true

CODE:

class Solution 
{
    public boolean isIsomorphic(String s, String t) 
    {
        if(s.length()!=t.length())
            return false;

        HashMap<Character,Character> mp=new HashMap<>();

        for(int i=0; i<s.length(); i++)
        {
            char a=s.charAt(i);
            char b=t.charAt(i);

            if(!mp.containsKey(a))
            {
                if(!mp.containsValue(b))
                    mp.put(a,b);

                else
                    return false;
            }

            else if(mp.get(a)!=b)
                return false;
        }
        return true;
    }
}