LeetCode 0387 - First Unique Character in a String

s = "leetcode"
return 0.

s = "loveleetcode",
return 2.

class Solution {
public:
    int firstUniqChar(const std::string& s) {
        auto book = std::vector<int>(26, false);
        for(int i = 0; i < s.size(); i++) {
            book[s[i] - 'a']++;
            }

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

        return -1;
    }
};