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.

思路：用一个hash表来存储两个字符串中字符的映射关系，当出现的字符已经在hash表中出现过，但映射关系与在hash表中原有的映射关系不同时，说明字符串不是同构的，但是也要注意万一出现两个字符映射到同一个字符，比如“ab” 和“aa” 中a和b映射同时映射到a，先把<a,a>放入hash表中，当遍历到b字符的时候，先判断hash表中有没有出现key为b的情况，很显然还没出现过，然后再判断hash表中value有没有出现过a，如果出现了说明已经有字符映射到a了，此时出现某个字符和b映射到同一个字符的情况，说明两个字符串不是同构的。

public class Solution {
public boolean isIsomorphic(String s, String t) {
HashMap<Character,Character> hs=new HashMap<Character,Character>();
int slength=s.length();
int tlength=t.length();
if(slength!=tlength)
return false;
for(int i=0;i<slength;i++)
{
if(hs.containsKey(s.charAt(i)))
{
char ch=hs.get(s.charAt(i));
if(ch!=t.charAt(i))
return false;
}
else if(hs.containsValue(t.charAt(i)))
return false;
else hs.put(s.charAt(i),t.charAt(i));
}
return true;
}
}