同构string

leetcode新题，另外似乎是L家面经题

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.

这里注意不能s中两个char map到t中相同char，所以同时还需要check当前t[i]是否已经对应其他char了，这样就需要两个map来保存map pair的关系

例如 ab, aa return false

 1  //use hashmap to record map of (s, t)
 2     public static boolean isIsomorphic(String s, String t) {
 3         if(s == null && t == null) {
 4             return true;
 5         }
 6         if(s.length() != t.length()) {
 7             return false;
 8         }
 9
10         HashMap<Character, Character> map1 = new HashMap<Character, Character>();//s[i], t[i]
11         HashMap<Character, Character> map2 = new HashMap<Character, Character>();//t[i], s[i]
12
13
14         for(int i = 0; i < s.length(); i++){
15             char schar = s.charAt(i);
16             char tchar = t.charAt(i);
17
18             if(!map1.containsKey(schar)){
19                 map1.put(schar, tchar);
20             }else if(map1.get(schar) != tchar) return false; // map to wrong char
21
22             //check no two map to same char in t
23             if(!map2.containsKey(tchar)){
24                 map2.put(tchar, schar);
25             }else if(map2.get(tchar) != schar) return false;
26         }
27
28         return true;
29     }