bzoj4800:[Ceoi2015]Ice Hockey World Championship

题意：有\(n\)个物品，\(m\)块钱，给定每个物品的价格，求买物品的方案数。
\(n<=40，m<=10^{18}\)
水题，\(meet\ in\ the\ middle\)裸题，随便搞搞，统计答案二分一下就没了

#include<cstdio>
#include<algorithm>
int n,sum;long long m,f[1500001],a[41],ans;
void dfs(int x,long long now){
	if(x>n/2){f[++sum]=now;return ;}
	dfs(x+1,now),dfs(x+1,now+a[x]);}
void dfs1(int x,long long now){
	if(x>n){ans+=(std::upper_bound(f+1,f+sum+1,m-now)-f)-1;return ;}
	dfs1(x+1,now),dfs1(x+1,now+a[x]);}
int main(){
	scanf("%d%lld",&n,&m);
	for(int i=1;i<=n;i++)scanf("%lld",&a[i]);
	dfs(1,0);std::sort(f+1,f+sum+1);dfs1(n/2+1,0);
	printf("%lld\n",ans);}