字符串3-字符串中的第一个唯一字符

题目网址：力扣

public class Solution {
    public int FirstUniqChar(string s) {
        int[] a = new int[26];
        for (int i = 0; i < s.Length; i++)
        {
            int index = (int)s[i];
            a[index-97] ++;
        }

        for(int i = 0; i < s.Length; i++)
        {
            if (a[(int)s[i]-97] == 1)
            {
                return i;
            }
        }
        return -1;
    }
}