IsomorphicStrings.java

package easy;

import java.util.HashMap;
import java.util.Map;

public class IsomorphicStrings {
    /**space should be O(1) since it only has alphabetic letters which are capped at 52.*/

    public boolean isIsomorphic(String s, String t) {
        if(s == null || s.length() == 0) return (t == null || t.length() == 0);
        if(t == null || t.length() == 0) return (s == null || s.length() == 0);
        char[] schar = s.toCharArray();
        char[] tchar = t.toCharArray();
        Map<Character, Character> map = new HashMap();
        if(s.length() != t.length()) return false;
        for(int i = 0; i < s.length(); i++){
            if(map.containsKey(schar[i])){
                if(map.get(schar[i]) != tchar[i]) return false;
            } else {
                if(map.containsValue(tchar[i])) return false;//this line is necessary for this case: ("ab", "aa")
                map.put(schar[i], tchar[i]);
            }
        }
        return true;
    }

}