HDU5887 Herbs Gathering(2016青岛网络赛 搜索 剪枝)

背包问题，由于数据大不容易dp，改为剪枝，先按性价比排序，若剩下的背包空间都以最高性价比选时不会比已找到的最优解更好时则剪枝，即

if(val + (LD)pk[d].val / (LD)pk[d].w * (lim - w) + EPS <= ans){
　　return;
}

没想到一发过，0ms

#include<cstdio>
#include<iostream>
#include<cstdlib>
#include<cstring>
#include<string>
#include<algorithm>
#include<map>
#include<queue>
#include<vector>
#include<cmath>
#include<utility>
using namespace std;
typedef long long LL;
typedef long double LD;

const int N = 108, INF = 0x3F3F3F3F, EPS = 0.4;

struct data{
	LL w, val;
	bool operator<(const data &tp)const{
		return val * tp.w > tp.val * w;
	}

}pk[N];


int n;
LL sum[N], sw[N], lim;
LL ans;

void dfs(int d, LL w, LL val){
	if(val > ans){
		ans =  val;
	}
	if(d >= n){
		return;
	}
	if(val + (LD)pk[d].val / (LD)pk[d].w * (lim - w) + EPS <= ans){
        return;
	}

	if(w + pk[d].w <= lim){
		dfs(d + 1, w + pk[d].w, val + pk[d].val);
	}
	dfs(d + 1, w, val);

}

int main(){
    while(~scanf("%d %I64d", &n, &lim)){
    	for(int i = 0; i < n; i++){
    		scanf("%I64d %I64d", &pk[i].w, &pk[i].val);
    	}
    	sort(pk, pk + n);
    	ans = 0;
    	sum[n] = 0;
    	sw[n] = 0;
    	dfs(0, 0, 0);
    	printf("%I64d
", ans);

    }
    return 0;
}