C++ Example

/**
 * Finds the first non-repeating character in a string.
 *
 * It is assumed that the string only consists of lower-case, English, characters.
 * This is a solution that only iterates the given string once, and uses O(1) additional
 * memory. The additional memory is O(1) because both the vector and map will hold a
 * maximum of 26 characters each. 
 *
 * @param s The string of characters to be analyzed.
 *
 * @returns The first non-repeated character, if found. Otherwise, '_'.
*/
char first_not_repeating_character(std::string s) {
    std::map<char, int> seen_chars;
    std::vector<char> order_seen;
    
    for (auto c: s) {        
        auto found = seen_chars.find(c);
        
        if (found != std::end(seen_chars)) {
            found->second += 1;
        } else {
            seen_chars[c] = 1;
            order_seen.push_back(c);
        }
    }
    
    for (auto c: order_seen)
        if (seen_chars[c] == 1) 
            return c;
    
    return '_';
}