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.

Solution:

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

思想：位置相同的字母占据同样的槽（512分为两个256槽，一共256个ASCII码值），相同位置的槽的值不同时即是两个字符串出现了不匹配的情况。