P2518 [HAOI2010]计数

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#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
#define re register
#define int long long
const int MAXn = 50;

int C[MAXn + 10][MAXn + 10];
void EvaC(int top) {
    for (re int i = 0; i <= top; ++i) {
        C[i][0] = C[i][i] = 1;
    }
    for (re int i = 1; i <= top; ++i) {
        for (re int j = 1; j < i; ++j) {
            C[i][j] = C[i - 1][j - 1] + C[i - 1][j];
        }
    }
}

int Pmult(int top, int *cnt, int sum = -1) {
    int ans = 1;
    if (sum == -1) {
        sum = 0;
        for (re int i = 1; i <= top; ++i) {
            sum += cnt[i];
        }
    }
    for (re int i = 1; i <= top; ++i) {
        if (cnt[i]) {
            ans *= C[sum][cnt[i]];
            sum -= cnt[i];
        }
    }
    return ans;
}

char str[MAXn + 10]; int len;
int sumcnt, cnt[11], ans;
signed main() {
    EvaC(MAXn);
    scanf("%s", str + 1);
    len = strlen(str + 1);
    reverse(str + 1, str + 1 + len);
    for (re int i = 1; i <= len; ++i) {
        if (str[i] - '0') {
            ++cnt[str[i] - '0'];
            ++sumcnt;
        }
    }
    for (re int i = len; i; --i) {
        if (str[i] - '0') {
            cnt[10] = i - 1 - sumcnt;
            ans += Pmult(10, cnt, i - 1);
            cnt[10] = i - 1 - (sumcnt - 1);
            for (re int j = 1; j < str[i] - '0'; ++j) {
                if (cnt[j]) {
                    --cnt[j];
                    ans += Pmult(10, cnt, i - 1);
                    ++cnt[j];
                }
            }
            --cnt[str[i] - '0'];
            --sumcnt;
        }
    }
    printf("%lld\n", ans);
}