387. First Unique Character in a

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.
给定一字符串，找出其中第一个不重复的字母，返回其索引，若不存在则返回-1，这里假定所有的字符都是小写字母。

思路
由于输入只含有小写字母，则可构建字母数组以统计每个字母出现的次数。

class Solution {
public:
    int firstUniqChar(string s) {
        int res=-1;
        int n=s.size();
        if(n==1)    return 0;
        if(n<=0)    return -1;
        int a[26]={0};
        for(int i=0;i<n;i++){
            a[s[i]-'a']++;
        }
        for(int i=0;i<n;i++){
            if(a[s[i]-'a']==1){
                res=i;
                break;
            }
        }
        return res;
    }
};