CH2401 送礼物 双向搜索

双向搜索：把前一半的可行状态搜出来，然后sort+unique，之后搜后一半时，结束时二分一下前一半的答案，拼出一个与W尽量接近的ans来更新

ps：距LYD说前一半取n/2+2时跑的最快。。。不知，，，

#include<iostream>
#include<cstdio>
#include<algorithm>
#define R register int
#define uint unsigned int
using namespace std;
inline int g() {
    R ret=; register char ch; while(!isdigit(ch=getchar()));
    do ret=ret*+(ch^); while(isdigit(ch=getchar())); return ret;
}
int n,m,hf,w[];
uint W,ans,a[(<<)+];
inline void dfs(int i,uint sum) {
    if(i==hf) {a[++m]=sum; return ;}
    dfs(i+,sum); if(sum+w[i]<=W) dfs(i+,sum+w[i]);
}
inline void dfs2(int i,uint sum) {
    if(i==n+) {
        R tmp=upper_bound(a+,a+m+,W-sum)-a-; //cout<<W-sum<<" "<<tmp<<" "<<a[tmp]<<endl;
        ans=max(ans,sum+a[tmp]); return;
    } dfs2(i+,sum); if(sum+w[i]<=W) dfs2(i+,sum+w[i]);
}
signed main() {
    W=g(),n=g();
    for(R i=;i<=n;++i) w[i]=g();
    sort(w+,w+n+,greater<int>());
    hf=(n>>)+; dfs(,); sort(a+,a+m+);
    m=unique(a+,a+m+)-a-;
    //for(R i=1;i<=40;++i) cout<<a[i]<<" "; cout<<endl;
    dfs2(hf,); printf("%d\n",ans);
}

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2019.04.26