public class Solution { //test case: "egg", "add" public boolean isIsomorphic(String s, String t) { //init check if(s==null || t==null) return false; if(s.length() != t.length()) return false; Map<Character, Character> map = new HashMap<Character, Character>(); Set<Character> set = new HashSet<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) return false; } else { if(set.contains(c2)) return false; else { map.put(c1, c2); set.add(c2); } } } return true; } }