IPv4

#include <iostream>
#include <algorithm>
using namespace std;

int main(){
    string s;    
    cin >> s;

    int a[26]={0,};
    int max = 0;
    int cnt;
    int index;

    transform(s.begin(),s.end(),s.begin(),::toupper);

    for(int i=0; i<s.length(); i++){
        a[s[i] - 'A']++;
    }

    for(int i=0; i<26; i++){
        if(max < a[i]){
            max = a[i];
            cnt = 0;
            index = i;
        }
        if(max == a[i]){
            cnt++;
        }
    }

    if(cnt > 1) cout << "?";
    else cout << (char)(index + 'A');
    
    return 0;
}

#include <iostream>
using namespace std;

int main(){
    int T,R,len;
    string s;
    cin >> T;
    for(int i=0; i<T; i++){
        cin >> R;
        cin >> s;
        for(int j = 0; j < s.length(); j++){
            for(int k = 0; k < R; k++){
                cout << s[j];
            }
        }
        cout << endl;
    }


}

#include <iostream>

using namespace std;

void show(string arr){
    string check = "abcdefghijklmnopqrstuvwxyz";
    for(int i=0; i < check.length(); i++){
            if(arr.find(check[i]) == string::npos) cout << -1 << " ";
            else{
                cout << arr.find(check[i]) << " ";
                continue;
            }
    }
}


int main(){
    string arr;

    cin >> arr;

    show(arr);

    return 0;
}

#include <iostream>
#define max 1000
using namespace std;

void cal(int n){
    int a,b,c,cnt=0;
    for(int i=100; i<=n; i++){
        a = i/100; //백의자리수
        b = (i%100)/10; //십의자리수
        c = (i%100)%10; //일의자리수
        if((a-b) == (b-c)) cnt++;
    }
    cout << 99+cnt << endl;
}

int main(){
    int n;
    cin >> n;

    if(n < 100) cout << n << endl;
    if(n > 99){
        cal(n);
    }
    return 0;
}

#include <iostream>
#include <cstdio>
#define max 10001

using namespace std;

bool arr[max];
int self(int n){
    int sum = n;

    while(1){
        if(n == 0) break;
        sum += n%10;    //한자리씩 더해줌
        n = n/10;   //한자리씩 없애줌
    }

    return sum;
}

int main(){

    for(int i=1; i<max; i++){
        int ind = self(i); // ind = 셀프넘버
        if(ind <= max){
            arr[ind] = true; // ind 가 arr의 인덱스로 들어가고 셀프넘버가 아니므로 true로 변경
        }
    }

    for(int i=1; i<max; i++){
        if(!arr[i]) cout << i << endl;
    }
    return 0;
}

#include <iostream>
#define max 1000
using namespace std;

void input(int n){
    float arr[max];
    
    for(int i=0; i<n; i++){
        double average = 0.0;
        int m,cnt=0;
        cin >> m;

        for(int j=0; j<m; j++){
            cin >> arr[j];
            average += arr[j];
        }
        average = average/m;
        
        for(int j=0; j<m; j++){
            if(arr[j] > average) cnt++;
        }
        cout<< fixed;
        cout.precision(3);
        cout << (double)cnt/m * 100.0 << "%" << endl;
    }
}

int main(){
    int n;
    cin >> n;

    input(n);

    return 0;
}

#include <iostream>

using namespace std;

void cal(int A,int B){
    int a = B/100;
    int b = (B-(a*100))/10;
    int c = B % 10;

    cout << A*c << endl;
    cout << A*b << endl;
    cout << A*a << endl;
    cout << A*B;
}

int main(){
    int A,B;

    cin >> A >> B;

    cal(A,B);

    return 0;
}

#include <iostream>

using namespace std;

void cal(int A,int B){
    cout << A+B << endl;
    cout << A-B << endl;
    cout << A*B << endl;
    cout << A/B << endl;
    cout << A%B << endl;
}

int main(){
    int A,B;
    cin >> A >> B;
    cal(A,B);
    return 0;
}

#include <iostream>
#include <algorithm>

#define MAX 1000

using namespace std;

class average{
    float n; //과목갯수
    float score[MAX]; //과목
    float M; //최고값
    float res; // 결과값

    public:
    average();
    void cal();
    void output();
};

average::average(){
    M = 0.0;
    res = 0.0;
}

void average::cal(){
    cin >> n;
    for(int i=0; i<n; i++){
        cin >> score[i];
        if(score[i] > M) M = score[i];
        res += score[i];
    }
    res = (res/M * 100.0) / n;
}

void average::output(){
    cout << res;
}

int main(){
    average student;

    student.cal();

    student.output();

    return 0;
}

너무 C만 쓰다보니까 최대한 C++ 스럽게 (?) 작성하려고 노력중이네요..