蓝桥试题 算法训练 摆动序列

#include <iostream>
#include <algorithm>
#include <cstring>
#include <cstdio>

#define ci cin.tie(0)
#define ios ios::sync_with_stdio(false)
#define fi first
#define se second

using namespace std;

typedef long long LL;
typedef pair<int, int> PII;

const int N = 22;

int n, ans; 
int a[N];
bool vis[N];

bool check(int num, int k)
{
    if (k == 1 || k == 2) return true;
    if (a[k - 1] > a[k - 2] && num < a[k - 2]) return true;
    if (a[k - 1] < a[k - 2] && num > a[k - 2]) return true;
    return false;
}

void dfs(int u)
{
    if (u > n) return ;
    for (int i = 1; i <= n; i ++ )
    {
        if (!vis[i] && check(i, u))
        {
            vis[i] = true;
            a[u] = i;
            if (u >= 2)
            {
                ans ++ ;
            /*    for (int i = 1; i <= u; i ++ )
                    cout << a[i] << ‘ ‘; 
                cout << endl; */
            } 
            dfs(u + 1);
            vis[i] = false;    
        }
    }
}

int main()
{
    ci;ios;
    cin >> n;
    dfs(1);
    cout << ans << endl;
    return 0;
}