【SSL2881】买汽水

买汽水

题目链接：SSLOJ/Luogu(个人题目)

---

解题思路

折半搜索

code

#include<algorithm>
#include<iostream>
#include<cstdio>
using namespace std;

int n,m;
int ans; 
int a[50];
int b[1100000],tot;

void find(int s)
{
	int l=0,r=tot;
	while(l<=r)
	{
		int mid=(l+r)/2;
		if(s+b[mid]<=m)
		{
			ans=max(ans,s+b[mid]);
			l=mid+1;
		}
		else
			r=mid-1;
	}
}

void dfs1(int now,int s)
{
	if(s>m)
		return;
	if(now>n/2)
	{
		b[++tot]=s;
		return;
	}
	dfs1(now+1,s);
	dfs1(now+1,s+a[now]);
}

void dfs2(int now,int s)
{
	if(s>m)
		return;
	if(now>n)
	{
		find(s);
		return;
	}
	dfs2(now+1,s);
	dfs2(now+1,s+a[now]);
}

int main()
{
	cin>>n>>m;
	for(int i=1;i<=n;i++)
		scanf("%d",&a[i]);
	dfs1(1,0);
	sort(b+1,b+tot+1);
	dfs2(n/2+1,0);
	cout<<ans<<endl;
}