【HDU 5887】Herbs Gathering（dfs+时间剪枝）

【HDU 5887】Herbs Gathering（dfs+时间剪枝）

题目大意：
n个物体 每个物品有体积和价值，取V体积问最大价值。

背包，不过数据很大……
然后……先写个暴力的dfs，TLE
然后按性价比排个序，TLE
然后加各种剪枝，TLE

赛后(clock()-st)/CLOCKS_PER_SEC <= 0.02 st为dfs前取的clock()
跑的比谁都快……不想说话。。

代码如下：

#include <iostream>#include <cmath>#include <vector>#include <cstdlib>#include <cstdio>#include <climits>#include <ctime>#include <cstring>#include <queue>#include <stack>#include <list>#include <algorithm>#include <map>#include <set>#define LL long long#define Pr pair<int,int>#define fread(ch) freopen(ch,"r",stdin)#define fwrite(ch) freopen(ch,"w",stdout)using namespace std;const int INF = 0x3f3f3f3f;const int msz = 10000;const int mod = 1e9+7;const double eps = 1e-8;struct Obj{    LL v,w;    double xjb;    bool operator < (const struct Obj a)const    {        return xjb > a.xjb;    }} obj[111];int n;LL v,mn;LL ans;double c;void dfs(int pos,LL sum,LL v){    if((clock()-c)/CLOCKS_PER_SEC > 0.02) return;    ans = max(ans,sum);    if(sum+obj[pos].xjb*v < ans) return;    if(v < mn) return;    if(pos == n) return;    if(obj[pos].v <= v) dfs(pos+1,sum+obj[pos].w,v-obj[pos].v);    dfs(pos+1,sum,v);}int main(){    //fread("");    //fwrite("");    while(~scanf("%d%lld",&n,&v))    {        mn = 1e9+7;        for(int i = 0; i < n; ++i)        {            scanf("%lld%lld",&obj[i].v,&obj[i].w);            obj[i].xjb = obj[i].w*1.0/obj[i].v;            mn = min(mn,obj[i].v);        }        sort(obj,obj+n);        ans = 0;        c = clock();        dfs(0,0,v);        printf("%lld\n",ans);    }    return 0;}