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  • hdu3586 Information Disturbing 【树形dp】

    题目链接

    hdu3586

    题解

    二分 + 简单的树形dp

    我正有练一下dp的必要了

    #include<iostream>
    #include<cstdio>
    #include<cmath>
    #include<cstring>
    #include<algorithm>
    #define LL long long int
    #define Redge(u) for (int k = h[u],to; k; k = ed[k].nxt)
    #define REP(i,n) for (int i = 1; i <= (n); i++)
    #define BUG(s,n) for (int i = 1; i <= (n); i++) cout<<s[i]<<' '; puts("");
    using namespace std;
    const int maxn = 1005,maxm = 100005,INF = 1000000000;
    inline int read(){
    	int out = 0,flag = 1; char c = getchar();
    	while (c < 48 || c > 57){if (c == '-') flag = -1; c = getchar();}
    	while (c >= 48 && c <= 57){out = (out << 3) + (out << 1) + c - 48; c = getchar();}
    	return out * flag;
    }
    int h[maxn],ne = 2;
    struct EDGE{int to,nxt,w;}ed[maxm];
    inline void build(int u,int v,int w){
    	ed[ne] = (EDGE){v,h[u],w}; h[u] = ne++;
    	ed[ne] = (EDGE){u,h[v],w}; h[v] = ne++;
    }
    int fa[maxn],w[maxn];
    int n,m,lim;
    void DFS(int u){
    	Redge(u) if ((to = ed[k].to) != fa[u]){
    		fa[to] = u; w[to] = ed[k].w;
    		DFS(to);
    	}
    }
    LL f[maxn][2];
    void dfs(int u){
    	if (w[u] <= lim) f[u][1] = w[u];
    	else f[u][1] = INF;
    	f[u][0] = 0;
    	int cnt = 0;
    	Redge(u) if ((to = ed[k].to) != fa[u]){
    		dfs(to); cnt++;
    		f[u][0] += min(f[to][0],f[to][1]);
    	}
    	if (!cnt) f[u][0] = INF;
    }
    bool check(int x){
    	lim = x;
    	dfs(1);
    	return f[1][0] <= m;
    }
    int main(){
    	while ((n = read()) && (m = read())){
    		memset(h,0,sizeof(h)); ne = 2;
    		int a,b,w;
    		for (int i = 1; i < n; i++){
    			a = read(); b = read(); w = read();
    			build(a,b,w);
    		}
    		DFS(1);
    		int l = 1,r = m,mid;
    		while (l < r){
    			mid = l + r >> 1;
    			if (check(mid)) r = mid;
    			else l = mid + 1;
    		}
    		if (check(l)) printf("%d
    ",l);
    		else puts("-1");
    	}
    	return 0;
    }
    
    
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  • 原文地址:https://www.cnblogs.com/Mychael/p/9009894.html
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