BZOJ2425 [HAOI2010]计数

比较简单的数位dp，但是要用到组合公式C，预处理吧。。。

 1 /**************************************************************
 2     Problem: 2425
 3     User: rausen
 4     Language: C++
 5     Result: Accepted
 6     Time:0 ms
 7     Memory:828 kb
 8 ****************************************************************/
 9
10 #include <cstdio>
11 #include <algorithm>
12 #include <cstring>
13
14 using namespace std;
15 typedef long long ll;
16 const int N = 55;
17 ll ans, c[N][N];
18 int num[10];
19 char s[N];
20 int n;
21
22 ll calc(int x){
23     ll res = 1;
24     for (int i = 0; i <= 9; ++i)
25         res *= c[x][num[i]], x -= num[i];
26     return res;
27 }
28
29 int main(){
30     scanf("%s", s + 1);
31     n = strlen(s + 1);
32     c[0][0] = 1;
33     for (int i = 0; i <= n; ++i)
34         for (int j = 0; j <= i; ++j)
35             c[i + 1][j] += c[i][j], c[i + 1][j + 1] += c[i][j];
36     for (int i = 1; i <= n; ++i)
37         ++num[s[i] - ‘0‘];
38     ans = 0;
39     for (int i = 1; i <= n; ++i){
40         for (int j = 0; j < s[i] - ‘0‘; ++j)
41             if (num[j]){
42                 --num[j];
43                 ans += calc(n - i);
44                 ++num[j];
45             }
46             --num[s[i] - ‘0‘];
47     }
48     printf("%lld\n", ans);
49     return 0;
50 }