IsomorphicWord.java

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

/**
 * @author humayun
 * http://www.programcreek.com/2014/05/leetcode-isomorphic-strings-java/
 * https://leetcode.com/problems/isomorphic-strings/
 * "egg" & "add" -> true [(e,a), (g,d), (g,d)]
 * "foo" & "bar" -> false [(f,b), (o,a), (o,r)]
 */
public class IsomorphicWord {
    public static void main(String[] args) {

        System.out.println(new IsomorphicWord().isIsomorphic("paper", "title"));
    }

    public boolean isIsomorphic(String s, String t) {
        if (s == null || t == null) {
            return false;
        }
        if (s.length() != t.length()) {
            return false;
        }
        if (s.length() == 0 && t.length() == 0) {
            return true;
        }

        Map<Character, Character> map = new HashMap<>();
        for (int i = 0; i < s.length(); i++) {
            Character c1 = s.charAt(i);
            Character c2 = t.charAt(i);

            Character c = getKey(map, c2);
            if (c != null & c != c1) {
                return false;
            } else if (map.containsKey(c1)) {
                if (c2 != map.get(c1)) {
                    return false;
                }
            } else {
                map.put(c1, c2);
            }
        }
        return true;
    }

    private Character getKey(Map<Character, Character> map, Character target) {
        for (Map.Entry<Character, Character> entry : map.entrySet()) {
            if (entry.getValue().equals(target)) {
                return entry.getKey();
            }
        }
        return null;
    }
}