SPOJ 1435 Vertex Cover 树形DP

i 表示节点 i ，j=0表示不选择其父节点，j=1表示选择其父节点。f 为其父节点。

取 每个节点选择/不选择 两者中较小的那个。

一组数据：

15
1 2
1 3
1 4
1 10
10 9
10 11
12 10
12 14
10 13
13 15
4 5
5 7
4 6
6 8

答案是6

#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <algorithm>
#include <vector>

using namespace std;

const int MAXN = 100010;

vector<int> adj[MAXN];
bool vis[MAXN][2];
int d[MAXN][2];
int n, m;
//int path[MAXN][2];
//int fang[MAXN];

int dp( int i, int j, int f )
{
    if ( vis[i][j] ) return d[i][j];
    vis[i][j] = true;
    int &ans = d[i][j];

    ans = 1;
    for ( int k = 0; k < (int)adj[i].size(); ++k )
    if ( adj[i][k] != f ) ans += dp( adj[i][k], 1, i );
    //path[i][j] = f;

    if ( j || f < 0 )
    {
        int sum = 0;
        for ( int k = 0; k < (int)adj[i].size(); ++k )
        if ( adj[i][k] != f )
           sum += dp( adj[i][k], 0, i );
        if ( sum < ans )
        {
            ans = sum;
            //path[i][j] = -f;
        }
    }
    return ans;
}

//void DFS( int i, int j, int f )
//{
//    if ( fang[i] != -1 ) return;
//    if ( !path[i][j] || path[i][j] == f )
//    {
//        if ( f >= 0 ) fang[f] = j;
//        for ( int k = 0; k < (int)adj[i].size(); ++k )
//            if ( adj[i][k] != f ) DFS( adj[i][k], 1, i );
//    }
//    else
//    {
//        if ( f >= 0 ) fang[f] = j;
//        for ( int k = 0; k < (int)adj[i].size(); ++k )
//            if ( adj[i][k] != f ) DFS( adj[i][k], 0, i );
//
//    }
//    return;
//}

int main()
{
    while ( ~scanf( "%d", &n ) )
    {
        m = n - 1;
        for ( int i = 0; i <= n; ++i ) adj[i].clear();

        for ( int i = 0; i < m; ++i )
        {
            int a, b;
            scanf( "%d%d", &a, &b );
            adj[a].push_back(b);
            adj[b].push_back(a);
        }

        memset( vis, false, sizeof(vis) );
        //memset( fang, -1, sizeof(fang) );
        //memset( path, 0, sizeof(path) );

        int ans = dp( 1, 0, -1 );
        printf( "%d
", ans );
    }
    return 0;
}