ACM 10062 -- Tell me the frequencies!

#include <iostream>

using namespace std;
void checkunique(string str);
bool first;

int main()
{
    string input;
    first = true;
    //while(cin>>input)
    while(getline(cin, input)){
        if(first == false)
            cout<<endl;
        checkunique(input);
        first = false;
    }
    return 0;
}


void checkunique(string str){
    int char_set[256] = {0};
    for(int i = 0 ; i < str.length() ; i ++){
        int val = str[i];
  char_set[val]++;

    }

    for(int i = 1 ; i <= str.length() ; i ++){
        for(int j = 256 ; j >= 0 ; j--){
            if(char_set[j] == i){
                cout<<j<<" "<<i<<endl;
            }
        }
    }
}