POJ 1947 Rebuilding Roads(树形DP)

题目链接

树形DP慢慢来，慢慢学习。这个题，就是树上的背包。

#include <cstdio>
#include <cstring>
#include <iostream>
#include <cmath>
#include <algorithm>
using namespace std;
#define INF 100000000
int dp[501][501];
int first[501];
int o[501],m,t;
struct node
{
    int u,v,next;
}edge[1000000];
void CL()
{
    t = 1;
    memset(first,-1,sizeof(first));
    memset(o,0,sizeof(o));
}
void add(int u,int v)
{
    edge[t].u = u;
    edge[t].v = v;
    edge[t].next = first[u];
    first[u] = t++;
}
void dfs(int x)
{
    int i,j,k,temp;
    for(i = 0;i <= m;i ++)
    dp[x][i] = INF;
    dp[x][1] = 0;
    for(i = first[x];i != -1;i = edge[i].next)
    {
        dfs(edge[i].v);
        for(j = m;j >= 1;j --)
        {
            temp = dp[x][j] + 1;
            for(k = 1;k < j;k ++)
            {
                temp = min(temp,dp[edge[i].v][j-k]+dp[x][k]);
            }
            dp[x][j] = temp;
        }
    }
}
int main()
{
    int n,i,u,v,ans;
    while(scanf("%d%d",&n,&m)!=EOF)
    {
        CL();
        for(i = 1;i < n;i ++)
        {
            scanf("%d%d",&u,&v);
            add(u,v);
            o[v] = 1;
        }
        for(i = 1;i <= n;i ++)
        {
            if(o[i] == 0)
            {
                dfs(i);
                ans = dp[i][m];
                break;
            }
        }
        for(i = 1;i <= n;i ++)
        {
            ans = min(ans,dp[i][m]+1);
        }
        printf("%d
",ans);
    }
    return  0;
}