[luogu4799 CEOI2015 Day2] 世界冰球锦标赛(折半搜索)

传送门

Solution

折半搜索裸题,注意(long long)

Code

#include <cmath>
#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <iostream>
#include <algorithm>
#define Re register 
#define Fo(i,a,b) for(Re int i=(a),_=(b);i<=_;i++)
#define Ro(i,a,b) for(Re int i=(b),_=(a);i>=_;i--)
using namespace std;
typedef long long LL;

inline LL read() {
	LL x=0,f=1;char c=getchar();
	while(!isdigit(c)) {if(c=='-')f=-f;c=getchar();}
	while(isdigit(c)) x=(x<<1)+(x<<3)+c-48,c=getchar();
	return x*f;
}

const int N=45;
LL n,m,k,t1,t2,ans;
LL da[N],s1[1050000],s2[1050000];

void dfs1(int pos,LL s) {
	if(s>m) return ;
	if(pos==k+1) {s1[++t1]=s;return ;} 
	dfs1(pos+1,s); dfs1(pos+1,s+da[pos]);
}

void dfs2(int pos,LL s) {
	if(s>m) return ;
	if(pos==n+1) {s2[++t2]=s;return ;}
	dfs2(pos+1,s); dfs2(pos+1,s+da[pos]);
}

int main() {
	n=read(),m=read();k=n/2;
	Fo(i,1,n) da[i]=read();
	dfs1(1,0);dfs2(k+1,0);
	sort(s1+1,s1+1+t1); sort(s2+1,s2+1+t2);
	LL p1=1,p2=t2;
	while(p1<=t1&&p2) {
		while(p2&&s1[p1]+s2[p2]>m) p2--;
		ans+=p2; p1++;
	}
	printf("%lld
",ans);
	return 0;
}