205. 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 gett.

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.

官方解答（不使用map）：

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;    }}