LeetCode Isomorphic Strings

原题链接在此：https://leetcode.com/problems/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.

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Hash Table

这道题可以用HashMap来做，但要注意的是当扫一个string遇到hm1中已有值时，要双向检测两个HashMap里的对应值，否则就会出现下列错误：

Input:"ab",
"aa"

Output:true

Expected:false
因为没有检查hm2，即使“aa”的第二个值 与 hm2中已有的a相同，也不会返回false。

AC Java:

public class Solution {
public boolean isIsomorphic(String s, String t) {
if(s.length() !=t.length())
return false;
if(s == null &&  t == null)
return true;

HashMap<Character,Character> hm1 = new HashMap<>();
HashMap<Character,Character> hm2 = new HashMap<>();

for(int i = 0;i< s.length();i++){
if(hm1.containsKey(s.charAt(i))){
if(hm1.get(s.charAt(i)) != t.charAt(i))
return false;

}
if(hm2.containsKey(t.charAt(i))){
if(hm2.get(t.charAt(i))!=s.charAt(i))
return false;
}

hm1.put(s.charAt(i),t.charAt(i));
hm2.put(t.charAt(i),s.charAt(i));

}
return true;
}
}