leetcode 242[easy]----Valid Anagram
难度：easy
Given two strings s and t, write a function to determine if t is an anagram of s.
For example,
s = "anagram", t = "nagaram", return true.
s = "rat", t = "car", return false.
Note:
You may assume the string contains only lowercase alphabets.
审题：判断两条string内的出现的字母是否相同且出现的次数相同。
这类题出现很多次了，依然用dictionary解题。