难度：easy
Given an arbitrary ransom note string and another string containing letters from all the magazines, write a function that will return true if the ransom note can be constructed from the magazines ; otherwise, it will return false.
Each letter in the magazine string can only be used once in your ransom note.
Note:
You may assume that both strings contain only lowercase letters.
思路：将两个str分别转字典，字典的key和value分别是”字母“和”字母出现的次数“，然后将两个字典作比较。如果ransomeNote形成的字典中所有的key都包含在magazine字典中，而且ran中key的value小于mag中的value，则magazine包含ransomeNote。