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public class Solution {
    public boolean isIsomorphic(String s, String t) {
        HashMap<Character, Character> map1 = new HashMap<Character, Character>();
        HashMap<Character, Character> map2 = new HashMap<Character, Character>();
        if(s == null || t == null || s.length() != t.length()) return false;
        int len = s.length();
        for(int i = 0; i < len; i++) {
            char temp1 = s.charAt(i);
            char temp2 = t.charAt(i);
            if(!map1.containsKey(temp1)) {
                map1.put(temp1, temp2);
            } else if (map1.get(temp1) != temp2){
                return false;
            }
            
            if(!map2.containsKey(temp2)) {
                map2.put(temp2, temp1);
            } else if (map2.get(temp2) != temp1){
                return false;
            }
        }
        return true;
    }
}

public class Solution {
    public boolean isIsomorphic(String s, String t) {
        
        if(s == null || t == null || s.length() != t.length()) return false;
        Set<Character> set1 = new HashSet<Character>();
        Set<Character> set2 = new HashSet<Character>();
        Map<Character,Character> map = new HashMap<Character,Character>();
        int len = s.length();
        for(int i = 0; i < len; i++) {
            char temp1 = s.charAt(i);
            char temp2 = t.charAt(i);
            set1.add(temp1);
            set2.add(temp2);
            map.put(temp1, temp2);
        }
        return set1.size() == set2.size() && set1.size() == map.size();
    }
}

/*
"add"
"egg"
"cd"
"aa"
*/

public class Solution {
    public boolean isIsomorphic(String s, String t) {
        return helper(s,t) && helper(t,s);
    }
    
    private boolean helper (String s, String t){
        if (s.length()!= t.length()){
            return false;
        }
        char [] hashmap = new char [256];
        for (int i = 0; i<s.length(); i++){
            char cS = s.charAt(i);
            char cT = t.charAt(i);
            if (hashmap[cS]==0){
                hashmap[cS] = cT;
            } else if (hashmap[cS] != cT){
                return false;
            }
        }
        return true;
    }
}

public class Solution {
    public boolean isIsomorphic(String s, String t) {
        char[] S = s.toCharArray();
        char[] T = t.toCharArray();

        if(S.length != T.length) return false;

        return isIsomorphic(S, T) ;
    }
    
    boolean isIsomorphic(char[] S, char[] T) {
        char[] MAP = new char[256];
        Set<Character> usedMapping = new HashSet<Character>();

        for(int i = 0; i < S.length; i++) {

            if(MAP[(int)S[i]] == 0) {
                // not mapped
                if (usedMapping.contains(T[i])) 
                    return false;
                MAP[(int)S[i]] = T[i];
                usedMapping.add(T[i]);
            } else {

                if ( MAP[(int)S[i]] != T[i]) {
                    return false;
                }
            }

        }

        return true;
    }
}