【搜索】【剪枝】bzoj1306 [CQOI2009]match循环赛

#include<cstdio>
using namespace std;
const int f[]={3,1,0,0};
int n,a[9],ans,Pre[9];
void dfs(int x,int y)
{
    if(Pre[x]>a[x])return;
    if(Pre[x]+(n-y+1)*3<a[x])return;
    if(x==n){ans++;return;}
    if(y==n)
      {
          int tmp=a[x]-Pre[x];
          if(tmp==2)return;
          Pre[y]+=f[tmp];
        dfs(x+1,x+2);
          Pre[y]-=f[tmp];
      }else{
    Pre[x]+=3;dfs(x,y+1);Pre[x]-=3;
    Pre[y]+=3;dfs(x,y+1);Pre[y]-=3;
    Pre[x]++;Pre[y]++;dfs(x,y+1);Pre[x]--;Pre[y]--;}
}
int main()
{
    scanf("%d",&n);
    for(int i=1;i<=n;i++)
      scanf("%d",&a[i]);
    dfs(1,2);
    printf("%d\n",ans);
    return 0;
}