POJ3659 Cell Phone Network

POJ3659 Cell Phone Network

Time Limit: 1000MS		Memory Limit: 65536K
Total Submissions: 3782		Accepted: 1305

Description

Farmer John has decided to give each of his cows a cell phone in hopes to encourage their social interaction. This, however, requires him to set up cell phone towers on his N (1 ≤N ≤ 10,000) pastures (conveniently numbered 1..N) so they can all communicate.

Exactly N-1 pairs of pastures are adjacent, and for any two pastures A and B (1 ≤ A ≤ N; 1 ≤ B ≤ N; A ≠ B) there is a sequence of adjacent pastures such that A is the first pasture in the sequence and B is the last. Farmer John can only place cell phone towers in the pastures, and each tower has enough range to provide service to the pasture it is on and all pastures adjacent to the pasture with the cell tower.

Help him determine the minimum number of towers he must install to provide cell phone service to each pasture.

Input

* Line 1: A single integer: N
* Lines 2..N: Each line specifies a pair of adjacent pastures with two space-separated integers: A and B

Output

* Line 1: A single integer indicating the minimum number of towers to install

Sample Input

5
1 3
5 2
4 3
3 5

Sample Output

2

Source

USACO 2008 January Gold

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题目大意：给定一棵树，求最小支配集。就是说用最少的点覆盖这棵树所有的边。

解题思路：发现，我的想法有点奇葩，和大家的一般思路不太一样。我对每个点设立了三种状态：0表示这个点没有覆盖正期待着被父亲节点覆盖，1表示在当前节点就是属于支配集，2表示当前节点被他的儿子覆盖。

状态转移：    dp[root][0]=sum{dp[son][2]};

　　　　　　dp[root][1]=sum{min{dp[son][0],dp[son][1],dp[son][2]}};

　　　　　　dp[root][2]=sum{min{dp[son][1],dp[son][2]}};这一条比较特殊，当括号内取小的时候如果一直取的是dp[son][2]，就需要特殊处理，就是说必须有一个点是dp[son][1]的取值。还有就是这个方法的边界处理比较麻烦。

#include <stdio.h>
#include <vector>
#include <string.h>
#define N 10005
#define INF 2000000
#define MIN(a,b) ((a)<(b)?(a):(b))
using namespace std;

vector<int>gra[N];
int fa[N],dp[N][3],n;

void dfs(int s,int f)
{
    fa[s]=f;
    dp[s][0]=0;
    dp[s][1]=1;
    dp[s][2]=0;
    if(gra[s].size()==1&&gra[s][0]==f)
    {
        dp[s][2]=INF;
        return ;
    }
    int mark=0;
    for(int i=0;i<gra[s].size();i++)
    {
        int t=gra[s][i];
        if(t==f)continue;
        dfs(t,s);
        dp[s][1]+=MIN(MIN(dp[t][0],dp[t][1]),dp[t][2]);
        dp[s][0]+=dp[t][2];
        if(dp[t][1]<=dp[t][2])
        {
            mark=1;
            dp[s][2]+=dp[t][1];
        }
        else
            dp[s][2]+=dp[t][2];
    }
    if(mark)return ;
    int temp=INF;
    for(int i=0;i<gra[s].size();i++)
    {
        int t=gra[s][i];
        if(t==f)continue;
        temp=MIN(temp,dp[s][2]+dp[t][1]-dp[t][2]);
    }
    if(temp!=INF)
        dp[s][2]=temp;
}

void re(void)
{
    scanf("%d",&n);
    memset(fa,0,sizeof(fa));
    memset(dp,0,sizeof(dp));
    for(int i=1;i<=n;i++)
        gra[i].clear();
    for(int i=1;i<n;i++)
    {
        int a,b;
        scanf("%d%d",&a,&b);
        gra[a].push_back(b);
        gra[b].push_back(a);
    }
}

void run(void)
{
    dfs(1,-1);
    printf("%d\n",MIN(dp[1][1],dp[1][2]));
}

int main()
{
    re();
    run();
}