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) {
        if (s == null && t == null) {
            return true;
        }
        HashMap<Character, Character> map1 = new HashMap<Character, Character>();
        HashSet<Character> set = new HashSet<Character>();
        for (int i = 0; i < s.length(); i++) {
            char c1 = s.charAt(i);
            char c2 = t.charAt(i);
            if (!map1.containsKey(c1)) {
                map1.put(c1, c2);
                if (set.contains(c2)) {
                    return false;
                }
                set.add(c2);
            } else {
                if (map1.get(c1) != c2) {
                    return false;
                }
            }
         
        }
        return true;
    }
}

===========

public class Solution {
    public boolean isIsomorphic(String s1, String s2) {
        int[] m = new int[512];
        for (int i = 0; i < s1.length(); i++) {
            if (m[s1.charAt(i)] != m[s2.charAt(i)+256]) return false;
            m[s1.charAt(i)] = m[s2.charAt(i)+256] = i + 1;
        }
        return true;
    }
}