题意

数论中的错排问题。记错排为Dn" role="presentation">DnDn,求Dnn!" role="presentation">Dnn!Dnn!。

分析

显然D1=0,D2=1" role="presentation">D1=0,D2=1D1=0,D2=1。当n≥3" role="presentation">n≥3n≥3时,不妨设n排在了第k位,其中k≠n" role="presentation">k≠nk≠n,也就是1≤k≤n−1" role="presentation">1≤k≤n−11≤k≤n−1。那么我们现在考虑k的情况。

  • 当k排在第n位时,n放k、k放n确定,因而除了n和k以外还有n-2个数,其错排数为Dn−2" role="presentation">Dn−2Dn−2。
  • 当k不排在第n位时,由于n在第k位,那么将第n位重新考虑成一个新的“第k位”,那么注意到“n在k”是确定的。剩下的排布方式同原来一致,只是k等价于n,因此:其错排数为Dn−1" role="presentation">Dn−1Dn−1。
    我们认真思考一下这个“重新考虑”是什么意思。 n在k,这是一个固定的条件,故这里不作考虑;而其他的元素必定不在各自的位置,k也是:它必定不在n!因而,这里相当于n−1" role="presentation">n−1n−1个元素的错排问题。

综上,当n" role="presentation">nn排在第k" role="presentation">kk位时共有Dn−2+Dn−1" role="presentation">Dn−2+Dn−1Dn−2+Dn−1种错排方法,又k" role="presentation">kk有从1" role="presentation">11到n−1" role="presentation">n−1n−1共n−1" role="presentation">n−1n−1种取法,我们可以得到:Dn=(n−1)(Dn−1+Dn−2)" role="presentation">Dn=(n−1)(Dn−1+Dn−2)Dn=(n−1)(Dn−1+Dn−2)

更多其他方法另请参见一位不愿具名的同学的题解

代码

#include <iostream>
#include <cstring>
#include <cstdio>
#include <iomanip>
#include <cstdlib> #define inf 0x3f3f3f3f
#define PB push_back
#define MP make_pair
#define fi first
#define se second
#define lowbit(x) (x & (-x))
#define rep(i, a, b) for (int i = (a); i <= (b); i++)
#define per(i, a, b) for (int i = (a); i >= (b); i--)
#define pr(x) cout << #x << " = " << x << " ";
#define prl(x) cout << #x << " = " << x << endl;
#define ZERO(X) memset((X), 0, sizeof(X))
#define ALL(X) (X).begin(), (X).end()
#define SZ(x) (int)x.size() using namespace std; typedef pair<int, int> PI;
typedef pair<pair<int, int>, int> PII;
typedef pair<pair<pair<int, int>, int>, int> PIII;
using ull = unsigned long long;
using ll = long long;
using ld = long double;
#define quickio \
ios::sync_with_stdio(false); \
cin.tie(0); \
cout.tie(0)
#define debug(...) fprintf(stderr, __VA_ARGS__), fflush(stderr)
/* debug("Precalc: %.3f\n", (double)(clock()) / CLOCKS_PER_SEC);
clock_t z = clock();
solve();
//debug("Test: %.3f\n", (double)(clock() - z) / CLOCKS_PER_SEC);
*/
template <typename T = int>
inline T read()
{
T val = 0, sign = 1;
char ch;
for (ch = getchar(); ch < '0' || ch > '9'; ch = getchar())
if (ch == '-')
sign = -1;
for (; ch >= '0' && ch <= '9'; ch = getchar())
val = val * 10 + ch - '0';
return sign * val;
}
int main()
{
ll arr[25];
arr[1] = 0;
arr[2] = 1;
ll jc[25];
jc[1] = 1;
jc[2] = 2;
for (int i = 3; i <= 20; ++i)
{
arr[i] = (i - 1) * (arr[i - 1] + arr[i - 2]);
jc[i] = jc[i - 1] * i;
} int T;
cin >> T;
while (T--)
{
int n;
cin >> n;
cout << fixed << setprecision(2)
<< double(arr[n]) * 100.0 / jc[n] << "%" << endl;
}
return 0;
}

修改于20180524,进一步方便自己回顾对错排问题的理解。

05-19 17:04