[LeetCode] 387. First Unique Character in a String

https://leetcode.com/contest/warm-up-contest/problems/first-unique-character-in-a-string/

Solution:
This is an easy level question. We may use the hash map to count the time of appearances for  each element. Then find the first element with count number of 1 (or the total number of this kind of element is 1). Since all the letters are in lower case, we can use a 26-letter long vector, instead of a hashmap. See code below.

Code:

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