[BZOJ4800][Ceoi2015][Meet In Middle]Ice Hockey World Championship

#include <cstdio>
#include <iostream>
#include <algorithm>
#include <vector>
#define N 45

using namespace std;

typedef long long ll;

int n;
ll m;
ll A
,B
;
vector<ll> pa,pb;

void dfs1(int x,ll w){
if(x>(n/2)) return (void)pa.push_back(w);
dfs1(x+1,w);
if(w+A[x]<=m) dfs1(x+1,w+A[x]);
}

void dfs2(int x,ll w){
if(x>(n+1)/2) return (void)pb.push_back(w);
dfs2(x+1,w);
if(w+B[x]<=m) dfs2(x+1,w+B[x]);
}

int main(){
scanf("%d%lld",&n,&m);
for(int i=1;i<=n/2;i++) scanf("%lld",&A[i]);
for(int i=1;i<=(n+1)/2;i++) scanf("%lld",&B[i]);
dfs1(1,0); dfs2(1,0);
sort(pa.begin(),pa.end()); sort(pb.begin(),pb.end());
//for(int i=0;i<pa.size();i++) printf("%lld ",pa[i]); putchar('\n');
//for(int i=0;i<pb.size();i++) printf("%lld ",pb[i]); putchar('\n');
ll Ans=0;
for(int i=0,j=pb.size()-1;i<pa.size();i++){
while(pa[i]>m-pb[j]&&j>=0) j--;
Ans+=j+1;
}
printf("%lld\n",Ans);
}