BZOJ 4800: [Ceoi2015]Ice Hockey World Championship

Description

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\(n\leqslant 40,m\leqslant 10^{18}\)。

Solution

meet in middle.

Code

/**************************************************************
    Problem: 4800
    User: BeiYu
    Language: C++
    Result: Accepted
    Time:5792 ms
    Memory:17676 kb
****************************************************************/
 
#include <bits/stdc++.h>
using namespace std;
 
typedef long long LL;
const int M = 45;
const int N = 1<<21;
 
int n,md,c;
LL m,ans;
LL a[M],b[N];
 
void DFS(int x,LL s) {
    if(s>m) return;
    if(x==md+1) { b[++c]=s;return; }
    DFS(x+1,s+a[x]),DFS(x+1,s);
}
void DDFS(int x,LL cur) {
    if(cur<0) return;
    if(x==n+1) { ans+=upper_bound(b+1,b+c+1,cur)-b-1;return; }
    DDFS(x+1,cur-a[x]),DDFS(x+1,cur);
}
 
int main() {
    cin>>n>>m;
    for(int i=1;i<=n;i++) cin>>a[i];
    md=(n>>1);
    DFS(1,0);
    sort(b+1,b+c+1);
    DDFS(md+1,m);
    cout<<ans<<endl;
    return 0;
}