POJ 1848 Tree

Tree

Time Limit: 1000ms

Memory Limit: 30000KB

This problem will be judged on PKU. Original ID: 1848
64-bit integer IO format: %lld      Java class name: Main

 

Consider a tree with N vertices, numbered from 1 to N. Add, if it is possible, a minimum number of edges thus every vertex belongs to exactly one cycle.

 

Input

The input has the following structure: 
N 
x(1) y(1) 
x(2) y(2) 
... 
x(N-1) y(n-1) 
N (3 <= N <=100) is the number of vertices. x(i) and y(i) (x(i), y(i) are integers, 1 <= x(i), y(i) <= N) represent the two vertices connected by the i-th edge. 

 

Output

The output will contain the value -1 if the problem doesn't have a solution, otherwise an integer, representing the number of added edges.

 

Sample Input

7
1 2
1 3
3 5
3 4
5 6
5 7

Sample Output

2

Source

Romania OI 2002

 

解题：树形dp

 

本题有三种状态，分别是
dp[u][0],以u为根，所有的点都在环内
dp[u][1],以u为根，除了u外其余的都在环内
dp[u][2],以u为根，除了u和与u点相连的链（至少有两个点）外，其余的点都在环内
有四种状态转移
1、所有子节点都满足在环内，只有根节点不在环内，
dp[u][1]=min(INF,sum);(sum是所有的dp[v][0]的和)
2、有一个子节点不在环内，以该子节点有一条链，将此链连到以u为根的树上,(+1)就可以使得u的所有子节点都在环内
dp[u][0]=min(dp[u][0],sum-dp[v][0]+dp[v][2]+1);
3、有一个子节点不在环内，其余自己处理，
dp[u][2]=min(dp[u][2],sum-dp[v][0]+min(dp[v][1],dp[v][2]));
4、有两个不在环内，将这两个加一条边连起来就可使得所有都在环内
dp[u][0]=min(dp[u][0],sum-dp[v][0]-dp[k][0]+min(dp[v][1],dp[v][2])+min(dp[k][1],dp[k][2])+1);

 

看来别人的解释，还是很好理解的

#include <iostream>
#include <cstdio>
#include <cstring>
#include <vector>
using namespace std;
const int INF = 10010;
const int maxn = 200;
vector<int>g[maxn];
int dp[maxn][3];
void dfs(int u,int fa){
    int sum = 0;
    dp[u][0] = dp[u][1] = dp[u][2] = INF;
    for(int i = g[u].size()-1; i >= 0; --i){
        if(g[u][i] == fa) continue;
        dfs(g[u][i],u);
        sum += dp[g[u][i]][0];
    }
    dp[u][1] = sum;
    for(int i = g[u].size()-1; i >= 0; --i){
        if(g[u][i] == fa) continue;
        dp[u][0] = min(dp[u][0],sum - dp[g[u][i]][0] + dp[g[u][i]][2] + 1);
        dp[u][2] = min(dp[u][2],sum - dp[g[u][i]][0] + min(dp[g[u][i]][1],dp[g[u][i]][2]));
        for(int j = g[u].size()-1; j >= 0; --j){
            if(g[u][j] != g[u][i] && g[u][j] != fa)
                dp[u][0] = min(dp[u][0],sum - dp[g[u][i]][0] - dp[g[u][j]][0]
                               + min(dp[g[u][i]][1],dp[g[u][i]][2]) + min(dp[g[u][j]][1],dp[g[u][j]][2]) + 1);
        }
    }
}
int main(){
    int n,u,v;
    while(~scanf("%d",&n)){
        for(int i = 0; i < maxn; ++i) g[i].clear();
        for(int i = 1; i < n; ++i){
            scanf("%d%d",&u,&v);
            g[u].push_back(v);
            g[v].push_back(u);
        }
        dfs(1,-1);
        printf("%d\n",dp[1][0] >= INF?-1:dp[1][0]);
    }
    return 0;
}