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.

public class Solution {
    public boolean isIsomorphic(String s, String t) {
        //We can define a map which tracks the char-char mappings. If a value is already mapped, it can not be mapped again.
        
       if(s == null || t == null)
       return false;
       
       if(s.length() != t.length())
       return false;
       
       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);
           
           if(map.containsKey(c1)){
               if(map.get(c1) != c2)// if not consistant with previous ones
               return false;
               
           }
           else{
               if (map.containsValue(c2))//if c2 is already being mapped
               return false;
               
               map.put(c1, c2);
           }
       }
       
       return true;
       
    }
}