Given a string, find the first non-repeating character in it and return it's index. If it doesn't exist, return -1.
Examples:
s = "leetcode"
return 0.
s = "loveleetcode",
return 2.
Note: You may assume the string contain only lowercase letters.
思路：找出第一个只出现一次的字母。利用字符索引，先遍历一遍数组统计，然后再遍历一次找出第一个索引值为1 的索引便是。
