URAL1389. Roadworks（dp)

1389

算个简单的树形DP吧 不知道是不是数据太水 竟然一A了 

就是对于当前节点有没有被选中就行选最优 有没有被选中的意思是有没有与它相连的边被选中 

#include <iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<stdlib.h>
#include<vector>
#include<cmath>
using namespace std;
#define N 100010
vector<int>d[N];
int dp[N][3],g;
struct node
{
    int u,v;
}p[N];
int dfs(int u,int v,int f)
{
    if(dp[v][f]!=-1)
    return dp[v][f];
    int i;
    int s[2][2] = {0};
    int flag =0,x=-1;
    for(i = 0 ; i < d[v].size() ; i++)
    {
        int tv = d[v][i];
        if(tv==u) continue;
        flag = 1;
        int s1 = dfs(v,tv,0);
        if(f)
        {
            s[(i+1)%2][0] = s[i%2][0]+s1;
        }
        else
        {
            int s2 = dfs(v,tv,1)+1;
            s[(i+1)%2][0] = s[i%2][0]+s1;
            if(s[i%2][1]+s1<s[i%2][0]+s2)
            {
                s[(i+1)%2][1] = s[i%2][0]+s2;
                x = tv;
            }
            else
            s[(i+1)%2][1] = s[i%2][1]+s1;
        }
    }
    if(!flag)
    {
        return dp[v][f] = 0;
    }
    if(x!=-1)
    {
        g++;
        p[g].u = v;
        p[g].v = x;
    }
    int maxz = 0;
    for(i = 0 ; i < 2 ; i++)
    maxz = max(maxz,max(s[0][i],s[1][i]));
    return dp[v][f] = maxz;
}
int main()
{
    int i,n,m;
    while(cin>>n>>m)
    {
        memset(dp,-1,sizeof(dp));
        for(i = 0 ; i <= n ;i++)
        d[i].clear();
        for(i = 1; i <= m ;i++)
        {
            int u,v;
            cin>>u>>v;
            d[u].push_back(v);
            d[v].push_back(u);
        }
        dfs(-1,1,0);
        cout<<dp[1][0]<<endl;
        for(i = 1; i <= g ; i++)
        cout<<p[i].u<<" "<<p[i].v<<endl;
    }
    return 0;
}