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

  1. 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

  1. void solve() {
  2.     string s;
  3.     cin >> s;
  4.     reverse(s.begin(), s.end());
  5.     for (int i = 0; i < s.size(); ++i) {
  6.         if (s[i] == '6')s[i] = '9';
  7.         else if (s[i] == '9')s[i] = '6';
  8.     }
  9.     cout << s;
  10. }

C - Made Up

给定三个数组，请问有多少种组合使得 AtCoder Beginner Contest 202 - 图1

简单的数组存数问题

  1. int a[1 << 17], b[1 << 17];
  2. void solve() {
  3.     int n; cin >> n;
  4.     for (int i = 1, x; i <= n; ++i) {
  5.         cin >> x;
  6.         a[x]++;
  7.     }
  8.     for (int i = 1; i <= n; ++i)cin >> b[i];
  9.     ll ans = 0;
  10.     for (int i = 1, x ; i <= n; ++i) {
  11.         cin >> x;
  12.         ans += a[b[x]];
  13.     }
  14.     cout << ans;
  15. }

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

  1. ll dp[61][61];
  2. void solve() {
  3.     int a, b; ll k;
  4.     cin >> a >> b >> k;
  5.     dp[0][0] = 1;
  6.     for (int i = 1; i <= a + b; ++i)
  7.         for (int j = 0; j <= i; ++j)
  8.             dp[i][j] = dp[i - 1][j] + (j ? dp[i - 1][j - 1] : 0);
  9.     a += b;
  10.     ll ans = 1;
  11.     while (a--) {
  12.         auto x = dp[a][b];
  13.         if (k < ans + x) cout << 'a';
  14.         else cout << 'b', ans += x, b--;
  15.     }
  16.     cout << "\n";
  17. }