AtCoder Beginner Contest 260 (A·B·C)

#include <bits/stdc++.h>
using namespace std;

const int N = 1000;

int a[N];

int main(){

    string s;

    cin >> s;

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

    for(int i = 'a'; i <= 'z'; i ++){
        if(a[i] == 1){
            cout << (char)i << endl;
            return 0;
        }
    }

    cout << -1 << endl;

    return 0;

}

#include <bits/stdc++.h>
using namespace std;

const int N = 1e6 + 3;

struct students{
    int m,e,p;
}stu[N];

int n,x,y,z; 

bool vis[N];

//对数学成绩排序
bool cmp1(students &s1,students &s2){
    if(s1.m == s2.m){
        return s1.p < s2.p;
    }
    else return s1.m > s2.m;
}

//对英语成绩排序
bool cmp2(students &s1,students &s2){
    if(s1.e == s2.e){
        return s1.p < s2.p;
    }
    else return s1.e > s2.e;
}

//对总成绩排序
bool cmp3(students &s1,students &s2){
    if(s1.m + s1.e == s2.m + s2.e){
        return s1.p < s2.p;
    }
    else return s1.m + s1.e > s2.m + s2.e;
}

int main(){

    cin >> n >> x >> y >> z;

    for(int i = 0; i < n; i ++){
        cin >> stu[i].m;
        stu[i].p = i + 1;
    }

    for(int i = 0; i < n; i ++) cin >> stu[i].e;

    if(x){
        int flag = 0;
        sort(stu,stu + n,cmp1);
        for(int i = 0; flag < x; i ++){
            if(!vis[stu[i].p]){
                vis[stu[i].p] = 1;
                flag++;
            }
        }
    }

    if(y){
        int flag = 0;
        sort(stu,stu + n,cmp2);
        for(int i = 0; flag < y; i ++){
            if(!vis[stu[i].p]){
                vis[stu[i].p] = 1;
                flag++;
            }
        }
    }

    if(z){
        int flag = 0;
        sort(stu,stu + n,cmp3);
        for(int i = 0; flag < z; i ++){
            if(!vis[stu[i].p]){
                vis[stu[i].p] = 1;
                flag++;
            }
        }
    }

    for(int i = 1; i <= n; i ++) if(vis[i]) cout << i << endl;

    return 0;

}

#include <bits/stdc++.h>
using namespace std;

typedef long long LL;

const int N = 20; 

LL n, x, y;

LL m = 1;

LL red[N], blue[N];

int main(){

    cin >> n >> x >> y;

    red[1] = 0, blue[1] = 1;

    for(int i = 2; i <= n; i ++){
        blue[i] = red[i - 1] + blue[i - 1] * y;
        red[i] = red[i - 1] + blue[i] * x;
    }

//  for(int i = 1; i <= n; i ++) cout << blue[i] << ' ' ;
//  cout << endl;

    cout << red[n] << endl;

    return 0;

}