北大算法基础
美丽栅栏 _练习代码

#include<iostream> 
#pragma warning(disable : 4996)//禁用scanf错误提示
using namespace std;

const int UP = 0;
const int DOWN = 1;
const int MAXN = 25;
long long c[MAXN][MAXN][2];
//c[i][k][DOWN] 是i根木棒以k木棒打头向下的合理方案数
//c[i][k][UP] 是i根木棒以k木棒打头向上的合理方案数
void Init(int n) {
	memset(c, 0, sizeof(c));
	c[1][1][UP] = c[1][1][DOWN] = 1;
	for(int i=2;i<=n;i++)
		for (int k = 1; k <= i; k++) {//枚举第一根木棒k
			for (int M = k ; M < i; M++)//枚举第二根木棒M比k长
				c[i][k][UP] += c[i - 1][M][DOWN];
			for (int N = 1; N <= k - 1; N++)
				c[i][k][DOWN] += c[i - 1][N][UP];//枚举第二根木棒N比k短
		}
}

void Print(int n, long long cc) {//n根木棒,第cc个序列
	long long skipped = 0;
	int seq[MAXN];//输出的序列
	int used[MAXN];//木棒是否用了
	memset(used, 0, sizeof(used));
	for (int i = 1; i <= n; i++) {//依次确定每个i位置的木棒
		int k = 0;
		int No = 0;//k在剩余木棒中是第几短
		for (k = 1; k <= n; k++) {
			skipped = 0;
			if (!used[k]) {//木棒没有被使用时执行
				No++;//k是未使用木棒中的第几根
				if (i == 1)
					skipped = c[n][No][UP] + c[n][No][DOWN];
				else {
					if (k > seq[i - 1])// && (i <= 2 || seq[i - 2] > seq[i - 1]))
						skipped = c[n - i + 1][No][DOWN];
					else if (k < seq[i - 1])// && (i <= 2 || seq[i - 2] < seq[i - 1]))
						skipped = c[n - i + 1][No][UP];
				}
				if (skipped >= cc)
					break;
				else
					cc -= skipped;
			}
		}
		used[k] = true;
		seq[i] = k;
	}
	for (int i = 1; i <= n; i++)
		printf("%d ", seq[i]);
	printf("\n");
}

int main() {
	int T, n;
	long long cc;
	Init(20);
	scanf("%d", &T);
	while (T--){
		scanf("%d %lld", &n, &cc);
		Print(n, cc);
	}
	return 0;
}