题目描述：

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.

解题思路：

用数组做映射。注意每次字典映射的时候赋值为i+1，这是为了防止跟第一次映射时候的赋值混淆。（因为数组的初始化赋值为0，若每次赋值为i，那么第一次赋值的时候也为0，这样就混淆了）。

代码如下：

public class Solution {

    public boolean isIsomorphic(String s, String t) {

        int[] mapS = new int[128];

        int[] mapT = new int[128];

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

        	if(mapS[s.charAt(i)] != mapT[t.charAt(i)])

        		return false;

        	mapS[s.charAt(i)] = mapT[t.charAt(i)] = i + 1;

        }

        return true;

    }

}