*Isomorphic string

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.

解法一：

public class Solution {
    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;
            }else{
                if(!map.containsValue(b))
                map.put(a,b);
                else return false;

            }
        }
        return true;

    }
}

reference：https://leetcode.com/discuss/36144/java-solution-using-hashmap

解法二：c++

class Solution {
public:
    bool isIsomorphic(string s, string t) {
        int m1[256] = {0}, m2[256] = {0}, n = s.size();
        for (int i = 0; i < n; ++i) {
            if (m1[s[i]] != m2[t[i]]) return false;
            m1[s[i]] = i + 1;
            m2[t[i]] = i + 1;
        }
        return true;
    }
};

reference：https://leetcode.com/discuss/33854/my-6-lines-solution