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.

一刷
题解：
第一次iter, 词频统计，首先用数组统计出每个character出现的次数。
第二次iter, 保证顺序，然后如果这次扫描到的character只出现过一次，返回。

class Solution {
    public int firstUniqChar(String s) {
        int[] freq = new int[26];
        for(int i=0; i<s.length(); i++){
            freq[s.charAt(i) - 'a']++;
        }
        for(int i = 0; i < s.length(); i ++){
            if(freq [s.charAt(i) - 'a'] == 1) return i;
        }
        return -1;
    }
}