POJ 1947 Rebuilding Roads（树形DP）

思路：dp[i][j]表示以i为根节点要得到j个结点的子树所需要切的最少边数，然后枚举切子树还是不切，转移就好了

#include<iostream>
#include<cstdio>
#include<cstring>
#include<vector>
using namespace std;
#define LL long long
#define INF 1e9
const int maxn = 200;
vector<int>e[maxn];
int fa[maxn];
int N,P;
int dp[maxn][maxn];

void dfs(int u,int f)
{
	for (int i = 0;i<=P;i++)
		dp[u][i]=INF;
    dp[u][1]=0;
	for(int i = 0;i<e[u].size();i++)
	{
		int v = e[u][i];
		if(v==f)
			continue;
		dfs(v,u);
        for(int j = P;j>=0;j--)
		{
			for(int k = 0;k<j;k++)
			{
				if(k)
					dp[u][j]=min(dp[u][j],dp[v][k]+dp[u][j-k]);
				else
					dp[u][j]+=1;
			}
		}
	}
}
void init()
{
	memset(fa,-1,sizeof(fa));
	for(int i = 0;i<=N;i++)
		e[i].clear();
}
int main()
{
    while(scanf("%d%d",&N,&P)!=EOF)
	{
		init();
		for(int i = 1;i<N;i++)
		{
            int u,v;
			scanf("%d%d",&u,&v);
			e[u].push_back(v);
			e[v].push_back(u);
			fa[u]=v;
		}
		int root;
        for(root = 1;fa[root]!=-1;root = fa[root]);
		dfs(root,-1);
        int ans = dp[root][P];
		for(int i = 1;i<=N;i++)
			ans = min(ans,dp[i][P]+1);
		printf("%d\n",ans);
	}
}

Description

The cows have reconstructed Farmer John's farm, with its N barns (1 <= N <= 150, number 1..N) after the terrible earthquake last May. The cows didn't have time to rebuild any extra roads, so now there is exactly one way to get from any given barn to any other barn. Thus, the farm transportation system can be represented as a tree. 

Farmer John wants to know how much damage another earthquake could do. He wants to know the minimum number of roads whose destruction would isolate a subtree of exactly P (1 <= P <= N) barns from the rest of the barns.

Input

* Line 1: Two integers, N and P 

* Lines 2..N: N-1 lines, each with two integers I and J. Node I is node J's parent in the tree of roads. 

Output

A single line containing the integer that is the minimum number of roads that need to be destroyed for a subtree of P nodes to be isolated. 

Sample Input

11 6
1 2
1 3
1 4
1 5
2 6
2 7
2 8
4 9
4 10
4 11

Sample Output

2

Hint

[A subtree with nodes (1, 2, 3, 6, 7, 8) will become isolated if roads 1-4 and 1-5 are destroyed.]