AtCoder Beginner Contest 090

A

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#include <bits/stdc++.h>

using namespace std;

const int N = 1e6 + 5;
typedef long long LL;
char a[3][3];
int main() {
    for (int i = 0; i < 3; i++) {
        for (int j = 0; j < 3; j++) {
            cin >> a[i][j];
        }
    }
    for (int i = 0; i < 3; i++) cout << a[i][i];
    cout << endl;
    return 0;
}

B

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#include <bits/stdc++.h>

using namespace std;

const int N = 1e6 + 5;
typedef long long LL;
int a, b;
bool check(int num)
{
    char s[10];
    int cnt = 0;
    while (num)
    {
        s[cnt++] = num % 10;
        num /= 10;
    }
    for (int i = 0; i < cnt; i++)
    {
        if (s[i] != s[cnt - 1 - i])
            return false;
    }
    return true;
}
int main()
{
    cin >> a >> b;
    int res = 0;
    for (int i = a; i <= b; i++)
    {
        if (check(i))
            res++;
    }
    cout << res << endl;
    return 0;
}

C

推式子，看哪些点点击多少次。

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#include <bits/stdc++.h>

using namespace std;

const int N = 1e6 + 5;
typedef long long LL;
LL a, b;
int main() {
    cin >> a >> b;
    if (a >= 2) a -= 2;
    if (b >= 2) b -= 2;
        cout << a * b << endl;
    return 0;
}

D

推式子（看题解去罢）

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// LUOGU_RID: 174587529
#include <bits/stdc++.h>
#define int long long
using namespace std;
int n, k, answer;
signed main()
{
    cin >> n >> k;
    for (int b = k + 1; b <= n; b++)
    {
        answer += (b - k) * (n / b);
        answer += max(0ll, n % b - k + 1);
        if (k <= 0)
            answer--;
    }
    cout << answer;
    return 0;
}