CF1183E/H Subsequences

 1 #include <bits/stdc++.h>
 2 using namespace std;
 3 typedef long long ll;
 4 ll dp[105][105], sum[105];
 5 int last[30];
 6 int main()
 7 {
 8     int n; ll m; string s;
 9     while (cin >> n >> m >> s)
10     {
11         memset(dp, 0, sizeof dp);
12         memset(sum, 0, sizeof sum);
13         memset(last, -1, sizeof last);
14         for (int i = 1; i <= n; i++)
15         {
16             dp[i][1] = 1;
17             for (int j = 2; j <= i; j++)
18             {
19                 for (int k = 0; k < 26; k++)
20                 {
21                     if (last[k] != -1)
22                     {
23                         int p = last[k];
24                         dp[i][j] += dp[p][j - 1];
25                     }
26                 }
27             }
28             last[s[i - 1] - ‘a‘] = i;
29         }
30         for (int i = 1; i <= n; i++)
31         {
32             for (int j = 0; j < 26; j++)
33             {
34                 if (last[j] != -1) sum[i] += dp[last[j]][i];
35             }
36         }
37         sum[0] = 1;
38         ll ans = 0;
39         bool flg = false;
40         for (int i = n; i >= 0; i--)
41         {
42             if (m - sum[i] <= 0) { ans += m * ((ll)n - i); flg = true; break; }
43             ans += sum[i] * ((ll)n - i); m -= sum[i];
44         }
45         cout << (flg ? ans : -1) << endl;
46     }
47     return 0;
48 }