class Solution {
public:
    int firstUniqChar(string s) {
        if(s.empty()) {
            return -1;
        }

        int hashmap[256] = {0};
        int cur = 0, next = 1;

        const int len = s.length();
        hashmap[s[cur]]++;

        while(next < len) {
            hashmap[s[next]]++;

            //>> if current char shows more than one time,
            //>> then find next char which shows at first time
            while(cur < len && hashmap[s[cur]] > 1) {
                ++cur;
            }

            if(cur == len) {
                return -1;
            }
            else if (hashmap[s[cur]] == 0) {
                //>> find a char which shows at first time
                hashmap[s[cur]]++;
                next = cur; //>> reset next pointer
            }

            ++next;
        }

        return cur;
    }
};