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。。。

/**************************************************************
    Problem: 2425
    User: rausen
    Language: C++
    Result: Accepted
    Time:0 ms
    Memory:828 kb
****************************************************************/
 
#include <cstdio>
#include <algorithm>
#include <cstring>
  
using namespace std;
typedef long long ll;
const int N = 55;
ll ans, c[N][N];
int num[10];
char s[N];
int n;
  
ll calc(int x){
    ll res = 1;
    for (int i = 0; i <= 9; ++i)
        res *= c[x][num[i]], x -= num[i];
    return res;
}
  
int main(){
    scanf("%s", s + 1);
    n = strlen(s + 1);
    c[0][0] = 1;
    for (int i = 0; i <= n; ++i)
        for (int j = 0; j <= i; ++j)
            c[i + 1][j] += c[i][j], c[i + 1][j + 1] += c[i][j];
    for (int i = 1; i <= n; ++i)
        ++num[s[i] - '0'];
    ans = 0;
    for (int i = 1; i <= n; ++i){
        for (int j = 0; j < s[i] - '0'; ++j)
            if (num[j]){
                --num[j];
                ans += calc(n - i);
                ++num[j];
            }
            --num[s[i] - '0'];
    }
    printf("%lld
", ans);
    return 0;
}