Leetcode #387 First Unique Character in a String

firstUniqChar.cpp

class Solution {
public:
    int firstUniqChar(string s) {
        if(s.length() == 0) return -1;
        
        vector<int> frequency(26, -1);
        
        for(int i = 0; i < s.length(); i++){
            if(frequency[s[i] - 'a'] == -1) frequency[s[i] - 'a'] = i;
            else{
                frequency[s[i] - 'a'] = -2;
            } 
        }
        
        for(int i = 0; i < s.length(); i++){
           if(frequency[s[i] - 'a'] >= 0) return i;
        }
        
        return -1;
    }
};