[BZOJ]4800: [Ceoi2015]Ice Hockey World Championship dfs+二分

题解：

代码：

#include<bits/stdc++.h>
using namespace std;
#define pa pair<int,int>
#define LL long long
const int Maxn=45;
const int inf=2147483647;
LL read()
{
LL x=0,f=1;char ch=getchar();
while(ch<'0'||ch>'9'){if(ch=='-')f=-1;ch=getchar();}
while(ch>='0'&&ch<='9'){x=(x<<3)+(x<<1)+(ch^48);ch=getchar();}
return x*f;
}
int n,lenA=0,lenB=0,lim;
LL m,a[Maxn],b[Maxn],A[1100000],B[1100000];
void dfs1(int x,LL t)
{
if(t>m)return;
if(x==lim+1){A[++lenA]=t;return;}
dfs1(x+1,t);dfs1(x+1,t+a[x]);
}
void dfs2(int x,LL t)
{
if(t>m)return;
if(x==lim+1){B[++lenB]=t;return;}
dfs2(x+1,t);dfs2(x+1,t+a[x]);
}
int work(LL x)
{
int l=1,r=lenA;
while(l<=r)
{
int mid=l+r>>1;
if(A[mid]+x<=m)l=mid+1;
else r=mid-1;
}return l-1;
}
int main()
{
n=read();m=read();
if(n<=20)
{
for(int i=1;i<=n;i++)a[i]=read();
lim=n;dfs1(1,0);
printf("%d",lenA);
}
else
{
for(int i=1;i<=n;i++)a[i]=read();
lim=20;dfs1(1,0);sort(A+1,A+1+lenA);
lim=n;dfs2(21,0);
LL ans=0;
for(int i=1;i<=lenB;i++)
{
if(B[i]+A[lenA]<=m)ans+=lenA;
else ans+=(LL)(work(B[i]));
}printf("%lld",ans);
}
}