Algorithm looks for the first character not repeating

Given a string s, find and return the first instance of a non-repeating character in it. If there is no such character, return '_'.

Example

• For s = "abacabad", the output should be
  firstNotRepeatingCharacter(s) = 'c'.
  There are 2 non-repeating characters in the string: 'c' and 'd'. Return c since it appears in the string first.
• For s = "abacabaabacaba", the output should be
  firstNotRepeatingCharacter(s) = '_'.
  There are no characters in this string that do not repeat.

Input/Output

• [time limit] 500ms (cpp)

• [input] string s
  A string that contains only lowercase English letters.
  Guaranteed constraints:
  1 ≤ s.length ≤ 105.
• [output] char
  The first non-repeating character in s, or '_' if there are no characters that do not repeat.

My this solution:

char firstNotRepeatingCharacter(std::string s)
{
    char temp = ' ';
    for (int i = 0; i < s.length(); i++) {
        for (int g = 0; g < s.length(); g++) {
            if (s.length() == 1) {
                temp = s[0];
            }
            else if (i == g) {
                continue;
            }
            else if (s[i] == s[g]) {
                temp = '_';
                break;
            }
            else {
                temp = s[i];
            }
        }
        if (temp != '_')
            return temp;
    }
    return temp;
}