205. 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.

题目含义：给定两个字符串s和t，确定它们是否是同构的。如果s中的元素被替换可以得到t，那么称这两个字符串是同构的。在用一个字符串的元素替换另一个字符串的元素的过程中，所有字符的顺序必须保留。

方法一：

 1     public boolean isIsomorphic(String s, String t) {
 2         int[] m1 = new int[256];
 3         int[] m2 = new int[256];
 4         int n = s.length();
 5         for (int i = 0; i < n; ++i) {
 6             int schar = s.charAt(i);
 7             int tchar = t.charAt(i);
 8             if (m1[schar] != m2[tchar]) return false;
 9             m1[schar] = i + 1;
10             m2[tchar] = i + 1;
11         }
12         return true;        
13     }

方法二：

 1     public boolean isIsomorphic(String s, String t) {
 2         if(s == null || s.length() <= 1) return true;
 3         HashMap<Character, Character> map = new HashMap<Character, Character>();
 4         for(int i = 0 ; i< s.length(); i++){
 5             char a = s.charAt(i);
 6             char b = t.charAt(i);
 7             if(map.containsKey(a)){
 8                 if(map.get(a).equals(b)) continue;
 9                 else return false; //一个a对应了多个b
10             }else{
11                 if(!map.containsValue(b)) map.put(a,b);
12                 else return false; //b出现了，但是a没有出现
13             }
14         }
15         return true;      
16     }