BZOJ3139: [Hnoi2013]比赛

题目：http://www.lydsy.com/JudgeOnline/problem.php?id=3139

记忆化搜索。用map来存当前点的状态。然后搜索顺序要从大到小搜。

#include<cstring>
#include<cstdio>
#include<iostream>
#include<algorithm>
#include<cmath>
#include<queue>
#include<map>
#define rep(i,l,r) for (int i=l;i<=r;i++)
#define down(i,l,r) for (int i=l;i>=r;i--)
#define clr(x,y) memset(x,y,sizeof(x))
#define inf int(1e9)
#define maxn 15
#define mm 1000000007
#define ll long long
using namespace std;
map<ll,ll >mp;
int a[maxn],tmp[maxn];
ll ans;
int n;
int read(){
    int x=0,f=1; char ch=getchar();
    while (!isdigit(ch)) {if (ch=='-') f=-1; ch=getchar();}
    while (isdigit(ch)) {x=x*10+ch-'0'; ch=getchar();}
    return x*f;
}
ll hash(int n){
    rep(i,1,n) tmp[i]=a[i];
    rep(i,1,n) rep(j,1,i) if (tmp[i]>tmp[j]) swap(tmp[i],tmp[j]); 
    ll res=n;
    rep(i,1,n) res+=res*28+tmp[i]%mm;
    return res; 
}
ll dfs(int o,int now){
    if (a[now]>(now-o)*3) return -1;
    if (o==now){
        if (now==1) return 1;
        else {
            ll h=hash(now-1);
            if (mp[h]) return mp[h];
            else return mp[h]=dfs(1,now-1);
        }
    }
    ll res=0,tmp;
    if (a[o]>=3){
        a[o]-=3; tmp=dfs(o+1,now);
        if (tmp!=-1) {res+=tmp; if (res>mm) res-=mm;}
        a[o]+=3;
    }    
    if (a[o]&&a[now]) {
        a[now]--; a[o]--; 
        tmp=dfs(o+1,now);
        if (tmp!=-1) {res+=tmp; if (res>mm) res-=mm;}
        a[o]++; a[now]++;
    }
    if (a[now]>=3){
        a[now]-=3;
        tmp=dfs(o+1,now);
        if (tmp!=-1) {res+=tmp; if (res>mm) res-=mm;}
        a[now]+=3;
    }
    if (!res) return -1; 
    return res;
}
int main(){
    n=read();
    rep(i,1,n) a[i]=read();
    rep(i,1,n) rep(j,1,i) if (a[i]>a[j]) swap(a[i],a[j]);     
    ans=dfs(1,n);
    printf("%lld\n",ans);
    return 0;
}