【题解】hdu3586 树形DP+二分

题目链接

#include<cstdio>
#include<vector>
#include<algorithm>
#include<cstring>
using namespace std;
const int INF=1100000;
int n,m;
const int N=1e3+10;
struct node{
    int to,cost;
};
vector<node>G[N];
int dp[N];
int used[N];
void dfs(int s,int cnt)
{
    int i,j,tmp;
    used[s]=1;
    for(i=0;i<G[s].size();i++)
    {
        tmp=G[s][i].to;
        if(!used[tmp])
        {
            dfs(tmp,cnt);
            if(G[s][i].cost>cnt)
            dp[s]+=dp[tmp];
            else
            dp[s]+=min(dp[tmp],G[s][i].cost);
        }
    }
}
int judge(int mid)
{
    int i;
    for(i=1;i<=n;i++)
    {
        if(G[i].size()==1&&i!=1)
        dp[i]=INF;
        else dp[i]=0;
    }
    memset(used,0,sizeof(used));
    dfs(1,mid);
    if(dp[1]<=m)
    return 1;
    return 0;
}
int main()
{
    //freopen("in.txt","r",stdin);
    while(scanf("%d%d",&n,&m)&&(n||m))
    {
        int l=1,ans=-1,r=0,i,a,b,w,u,v;
        for(i=0;i<=n;i++)
        G[i].clear();
        for(i=0;i<n-1;i++)
        {
            scanf("%d%d%d",&u,&v,&w);
            G[u].push_back((node){v,w});
            G[v].push_back((node){u,w});
            r=max(r,w);//二分上限 
        }

        while(l<=r)
        { 
            int mid=(l+r)>>1;//写在里面 
            if(judge(mid))
            {
                ans=mid;
                r=mid-1;
            }
            else l=mid+1;
        }
        printf("%d\n",ans);
    }
    return 0;
}

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转载自blog.csdn.net/qq_41958841/article/details/81636862