[BZOJ 4800][Ceoi2015]Ice Hockey World Championship（Meet-in-the-Middle）

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。

Solution

敲一道搜索水题吧…

Meet-in-the-Middle思想，分成两部分搜

#include<iostream>
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<algorithm>
#include<vector>
typedef long long LL;
using namespace std;
int n;
LL m,w[45];
vector<LL>v[2];
void dfs(int x,int y,LL c,int f)
{
    if(x>y)
    {v[f].push_back(c);return;}
    if(c+w[x]<=m)dfs(x+1,y,c+w[x],f);
    dfs(x+1,y,c,f);
}
int main()
{
    scanf("%d%lld",&n,&m);
    for(int i=1;i<=n;i++)
    scanf("%lld",&w[i]);
    dfs(1,n/2,0,0);
    dfs(n/2+1,n,0,1);
    sort(v[0].begin(),v[0].end());
    sort(v[1].begin(),v[1].end());
    int i=0,j=v[1].size()-1;
    LL ans=0;
    while(i<v[0].size()&&j>=0)
    {
        while(v[0][i]+v[1][j]>m&&j>=0)j--;
        ans+=j+1;
        i++;
    }
    printf("%lld\n",ans);
    return 0;
}