Given a string, find the first non-repeating character in it and return it's index. If it doesn't exist, return -1.
Examples:
Note: You may assume the string contain only lowercase letters.
这道题确实没有什么难度，我们只要用哈希表建立每个字符和其出现次数的映射，然后按顺序遍历字符串，找到第一个出现次数为1的字符，返回其位置即可，参见代码如下：