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.
（一个字符与另一个字符的替换一定是按照字符的顺序发生的。两个不同的字符不能同时映射到同一个字符，但是一个字符可以映射到它自己。）
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.
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题目分析：判断两个字符串是否同构。
解题思路：使用两个hashmap，map和map1一个放置s到t的映射，另一个放置t到s的映射，循环遍历两个字符串的每个字符，key和value分别等于s和t的第i个字符，如果map中已经存在当前的key，则需要判断value是否与map中取出的值是否相等，如果不相等直接return false；
这样就确保了s到t同一个字符不会映射到不同的字符上。同理，map1存放t到s的映射，确保t到s同一个字符不会映射到不同的字符上.
代码：
1 public class Solution {
2 public static boolean isIsomorphic(String s, String t) {
3
4 if(null==s&&null==t){
5
6 return true;
7 }
8 if(s.length()!=t.length()){
9
10 return false;
11 }
12 //循环遍历两个字符串的每个字符，把s的每个字符当做键,t中的每个字符当做值，
13 //每次放入map之前，检查是否存在键，若存在，检查值是否相等，若相等，继续，若不等，返回false
14 Map<Character,Character> map=new HashMap<Character,Character>();
15 Map<Character,Character> map1=new HashMap<Character,Character>();
16 char key;
17 char value;
18 for(int i=0;i<s.length();i++){
19 key=s.charAt(i);
20 value=t.charAt(i);
21
22 if(map.containsKey((Character)key)){
23 if(value!=map.get((Character)key)){
24 return false;
25 }
26 }else{
27 if(map1.containsKey(value)){
28 if(key!=map1.get((Character)value)){
29 return false;
30 }
31 }
32 map.put(key,value);
33 map1.put(value, key);
34 }
35 }
36 return true;
37 }
38 }