題目：
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.
假定s和t的長度一樣。

方法一：效能49ms

class Solution(object):
    def isIsomorphic(self, s, t):
        """
        :type s: str
        :type t: str
        :rtype: bool
        """
 

        hashmap = {}
        mapval = {}
        for i in xrange(len(s)):
            if s[i] in hashmap:
                if hashmap[s[i]] != t[i]:
                    return False
            elif t[i] in mapval:
                    return False
            else:
                hashmap[s[i]] = t[i]
                mapval[t[i]] = True
 

        return True

這個演算法是我已知的效能最好的了。。