[POJ 1947]Rebuilding Roads (树形dp)

 1 import java.util.*;
 2 import static java.lang.Math.*;
 3 
 4 
 5 class Graph{
 6     
 7     ArrayList<ArrayList<Integer>> e;
 8     int [][] dp;
 9     int [] sum;
10     boolean [] vis;
11     final static int INF = 99999999;
12     
13     public Graph(int n){
14         e = new ArrayList<ArrayList<Integer>>();
15         for(int i=0;i<=n;i++){
16             e.add(new ArrayList<Integer>());
17         }        
18         dp = new int[n+1][n+1];
19         for(int i=0;i<n+1;i++){
20             Arrays.fill(dp[i],INF);
21         }
22         sum = new int[n+1];
23         vis = new boolean[n+1];
24     }
25     
26     public void add_edge(int from,int to){
27         ArrayList<Integer> t = e.get(from);
28         t.add(to);
29     }
30     
31     public void dfs(int u){
32         if( vis[u] ) return;
33         vis[u] = true;
34         int tot = 0;
35         sum[u] = 1;
36         ArrayList<Integer> t = e.get(u);
37         
38         for(int v:t){
39             if( !vis[v] ){
40                 tot++;
41                 dfs(v);            
42                 sum[u] += sum[v];
43             }            
44         }
45         
46         if( tot==0 ) {
47             dp[u][1] = 0;
48             return;
49         }
50         
51         dp[u][1] = tot;
52         for(int v:t){
53             for(int i=sum[u];i>=1;i--){
54                 for(int j=1;j<=sum[v];j++){
55                     if( i+j <= sum[u] )
56                         dp[u][i+j] = min(dp[u][i+j],dp[u][i]+dp[v][j]-1);
57                 }
58             }
59         }
60     }
61     
62     public int get_ans(){
63         int ans = dp[1][Main.P];
64         for(int i=2;i<=Main.N;i++){
65             ans = min(ans,dp[i][Main.P]+1);
66         }
67         return ans;
68     }
69     
70 }
71 
72 
73 public class Main{
74     static int N,P;
75     static Scanner sc = new Scanner(System.in);
76     public static void main(String[] args){
77         while( sc.hasNext() ){
78             N = sc.nextInt();
79             P = sc.nextInt();
80             
81             Graph G = new Graph(N);
82             
83             for(int i=1;i<N;i++){
84                 int a = sc.nextInt();
85                 int b = sc.nextInt();
86                 G.add_edge(a, b);                
87             }
88             
89             G.dfs(1);
90             
91             //System.out.println(G.dp[1][P]);
92             int ans = G.get_ans();
93             System.out.println(ans);
94         }
95     }
96 }