AISing Programming Contest 2021（AtCoder Beginner Contest 202） 简单题解记录

补题链接：Here

A - Three Dice

水题，问给定三次摇色子的正面，请问3次结果以后相对面的点数和

cout << (21 - a - b - c) << "\n";

B - 180°

水题，首先反转字符串，然后遇到 6 或者9 则相互替代即可

void solve() {
    string s;
    cin >> s;
    reverse(s.begin(), s.end());
    for (int i = 0; i < s.size(); ++i) {
        if (s[i] == '6')s[i] = '9';
        else if (s[i] == '9')s[i] = '6';
    }
    cout << s;
}

C - Made Up

给定三个数组，请问有多少种组合使得 $A_i=B_{c_i}$

简单的数组存数问题

int a[1 << 17], b[1 << 17];
void solve() {
    int n; cin >> n;
    for (int i = 1, x; i <= n; ++i) {
        cin >> x;
        a[x]++;
    }
    for (int i = 1; i <= n; ++i)cin >> b[i];
    ll ans = 0;
    for (int i = 1, x ; i <= n; ++i) {
        cin >> x;
        ans += a[b[x]];
    }
    cout << ans;
}

D - aab aba baa

这道题的写法参考了一下 繁凡さん的题解：here

一开始没想到 DP 辅助，写了半天的纯组合问题

ll dp[61][61];
void solve() {
    int a, b; ll k;
    cin >> a >> b >> k;
    dp[0][0] = 1;
    for (int i = 1; i <= a + b; ++i)
        for (int j = 0; j <= i; ++j)
            dp[i][j] = dp[i - 1][j] + (j ? dp[i - 1][j - 1] : 0);
    a += b;
    ll ans = 1;
    while (a--) {
        auto x = dp[a][b];
        if (k < ans + x) cout << 'a';
        else cout << 'b', ans += x, b--;
    }
    cout << "\n";
}