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：

    public boolean isIsomorphic(String s, String t) {
        int[] m1 = new int[256];
        int[] m2 = new int[256];
        int n = s.length();
        for (int i = 0; i < n; ++i) {
            int schar = s.charAt(i);
            int tchar = t.charAt(i);
            if (m1[schar] != m2[tchar]) return false;
            m1[schar] = i + 1;
            m2[tchar] = i + 1;
        }
        return true;        
    }

方法二：

    public boolean isIsomorphic(String s, String t) {
        if(s == null || s.length() <= 1) return true;
        HashMap<Character, Character> map = new HashMap<Character, Character>();
        for(int i = 0 ; i< s.length(); i++){
            char a = s.charAt(i);
            char b = t.charAt(i);
            if(map.containsKey(a)){
                if(map.get(a).equals(b)) continue;
                else return false; //一个a对应了多个b
            }else{
                if(!map.containsValue(b)) map.put(a,b);
                else return false; //b出现了，但是a没有出现
            }
        }
        return true;      
    }