HDU 3586-Information Disturbing（树形dp）

#include <map>
#include <set>
#include <list>
#include <cmath>
#include <queue>
#include <stack>
#include <cstdio>
#include <vector>
#include <string>
#include <cctype>
#include <complex>
#include <cassert>
#include <utility>
#include <cstring>
#include <cstdlib>
#include <iostream>
#include <algorithm>
using namespace std;
typedef pair<int,int> PII;
typedef long long ll;
#define lson l,m,rt<<1
#define pi acos(-1.0)
#define rson m+1,r,rt<<11
#define All 1,N,1
#define read freopen("in.txt", "r", stdin)
const ll  INFll = 0x3f3f3f3f3f3f3f3fLL;
const int INF=1<<20;
const int mod =  1000000007;
struct edge{
    int t,w;
};
vector<edge>e[1010];
int dp[1010],n,m,used[1010];
void dfs(int root,int limit){
    used[root]=1;
    int f=0,tmp=0;
    for(int i=0;i<e[root].size();++i){
        edge d=e[root][i];
        int son=d.t;
        int c=d.w;
        if(used[son])continue;
        f=1;
        dfs(son,limit);
        if(c<=limit){
            tmp+=min(dp[son],c);//小于上限，可切可不切
        }
        else tmp+=dp[son];//大于上限不能切断
    }
    if(f)
    dp[root]=tmp;
    else
    dp[root]=INF;
}
int main()
{
    while(~scanf("%d%d",&n,&m)){
        if(n==0&&m==0)break;
        int a;
        for(int i=1;i<=n;++i)
            e[i].clear();
        for(int i=0;i<n-1;++i){
            edge b;
            scanf("%d%d%d",&a,&b.t,&b.w);
            e[a].push_back(b);
            int tt=b.t;
            b.t=a;
            e[tt].push_back(b);
        }
        int l=0,r=m,ff=-1;
        while(l<=r){
            memset(dp,0,sizeof(dp));
            memset(used,0,sizeof(used));
            int mid=(l+r)>>1;
            dfs(1,mid);
            if(dp[1]<=m){r=mid-1;ff=mid;}
            else l=mid+1;
        }
        printf("%d\n",ff);
    }
return 0;
}