Find duplicate characters in a string c++ code example

Example 1: count occurrences of character in string c++

std::string s = "a_b_c";
size_t n = std::count(s.begin(), s.end(), '_'); // n=2

Example 2: finding no of unique characters in a string c++

int countDistinct(string s) 
{ 

    unordered_map<char, int> m; 

    for (int i = 0; i < s.length(); i++) { 
        m[s[i]]++; 
    } 

    return m.size(); 
}