poj之旅——3977

题目描述：给出n个数，求一个非空子集绝对值尽量小。

题解：非常明显的折半枚举。必须用map做，我原来用数组和vector，结果TLE了。。。

       逼着我用奇怪的迭代器。

参考程序：

#include<cstdio>
#include<algorithm>
#include<map>
#include<limits>
#include<iostream>
#define maxn 500
using namespace std;
typedef long long LL;
LL Abs(const LL& x){
	return x>=0?x:-x;
}
LL a[maxn];
int n;
int main(){
	while (cin>>n && n){
		map<LL,int>b;
		for (int i=0;i<n;i++)cin>>a[i];
		pair<LL,int>result(Abs(a[0]),1);
		int half=n/2;
		for (int i=1;i<1<<half;i++){
			LL sum=0;
			int ct=0;
			for (int j=0;j<half;j++)
				if (i>>j &1){
					sum+=a[j];
					ct++;
				}
			result=min(result,make_pair(Abs(sum),ct));
			map<LL,int>::iterator it=b.find(sum);
			if (it != b.end())it->second=min(it->second,ct);
				else b[sum]=ct;
		}
		for (int i=1;i<1<<(n-half);i++){
			LL sum=0;
			int ct=0;
			for (int j=0;j<n-half;j++)
				if (i>>j &1){
					sum+=a[half+j];
					ct++;
				}
			result=min(result,make_pair(Abs(sum),ct));
			map<LL,int>::iterator it=b.lower_bound(-sum);
			if (it!=b.end())result=min(result,make_pair(Abs(it->first+sum),it->second+ct));
			if (it!=b.begin()){
				it--;
				result=min(result,make_pair(Abs(it->first+sum),it->second+ct));	
			}
		}
		cout<<Abs(result.first)<<" "<<result.second<<endl;
	}
	return 0;
}

给出vector和array的，留作纪念：

vector：

#include<cstdio>
#include<algorithm>
#include<map>
#include<vector>
#define maxn 50
using namespace std;
typedef long long LL;
LL Abs(const LL& x){
	return x>=0?x:-x;
}
LL a[maxn];
vector<LL> c;
int n;
void Min(LL &res,int &ans,LL sum,int ct){
	if (sum>res)return;
	if (res>sum){
		res=sum;
		ans=ct;
		return;
	}else ans=min(ans,ct);
}
int main(){
	while (scanf("%d",&n)==1 && n){
		map<LL,int>b;
		for (int i=0;i<n;i++)scanf("%lld",&a[i]);
		LL res=a[0];
		int ans=1;
		int half=n/2;
		for (int i=1;i<1<<half;i++){
			LL sum=0;
			int ct=0;
			for (int j=0;j<half;j++)
				if (i>>j &1){
					sum+=a[j];
					ct++;
				}
			Min(res,ans,Abs(sum),ct);
			c.push_back(sum);
			b[sum]=min(b[sum],ct);	
		}
		sort(c.begin(),c.end());
		for (int i=1;i<1<<(n-half);i++){
			LL sum=0;
			int ct=0;
			for (int j=0;j<n-half;j++)
				if (i>>j &1){
					sum+=a[half+j];
					ct++;
				}
			Min(res,ans,Abs(sum),ct);
			int loc=lower_bound(c.begin(),c.end(),-sum)-c.begin();
			if (loc!=c.size())Min(res,ans,Abs(c[loc]+sum),b[c[loc]]+ct);
			if (loc!=0)Min(res,ans,Abs(c[loc-1]+sum),b[c[loc-1]]+ct);	
		}
		printf("%lld %d\n",Abs(res),ans);
	}
	return 0;
}

array：

#include<cstdio>
#include<algorithm>
#include<map>
#include<vector>
#define maxn 50
#define expn 1<<20
using namespace std;
typedef long long LL;
LL Abs(const LL& x){
	return x>=0?x:-x;
}
LL a[maxn];
LL c[expn];
int n;
void Min(LL &res,int &ans,LL sum,int ct){
	if (sum>res)return;
	if (res>sum){
		res=sum;
		ans=ct;
		return;
	}else ans=min(ans,ct);
}
int main(){
	while (scanf("%d",&n)==1 && n){
		map<LL,int>b;
		for (int i=0;i<n;i++)scanf("%lld",&a[i]);
		LL res=a[0];
		int ans=1;
		int half=n/2;
		int cnt=0;
		for (int i=1;i<1<<half;i++){
			LL sum=0;
			int ct=0;
			for (int j=0;j<half;j++)
				if (i>>j &1){
					sum+=a[j];
					ct++;
				}
			Min(res,ans,Abs(sum),ct);
			c[cnt++]=sum;
			b[sum]=min(b[sum],ct);	
		}
		sort(c,c+cnt);
		for (int i=1;i<1<<(n-half);i++){
			LL sum=0;
			int ct=0;
			for (int j=0;j<n-half;j++)
				if (i>>j &1){
					sum+=a[half+j];
					ct++;
				}
			Min(res,ans,Abs(sum),ct);
			int loc=lower_bound(c,c+cnt,-sum)-c;
			Min(res,ans,Abs(c[loc]+sum),b[c[loc]]+ct);
			if (loc!=0)Min(res,ans,Abs(c[loc-1]+sum),b[c[loc-1]]+ct);	
		}
		printf("%lld %d\n",Abs(res),ans);
	}
	return 0;
}

//坑爹的poj，调了2个小时。。。。