HDU

HDU - 1054

题意：就是求最小顶点覆盖。树形dp过了的题，看着是说怎么这么眼熟

思路：这个二分图比较好想的，找个根，与根的最短距离为奇数的点丢在一部分，是偶数的点丢在另外一部分，然后连边求边的匹配。

讲真我现在看到一些图还是觉得懵，不知道如何建图...今天看网络流好不容易把那个最基础的算法弄懂了点点，想要看一下题，结果第一题就不知道怎么建图....有点绝望....

#include <cstdio>#include <cstring>#include <iostream>#include <algorithm>using namespace std;int n;const int maxn = 2000,maxe = 6e3;struct node{    int to,next;    node(){}    node(int a,int b){to = a; next = b;}}edge[maxe];int edgenum;int h[maxn],d[maxn],in[maxn],out[maxn],used[maxn],belong[maxn];void init(){    for(int i = 0; i < maxn; i++)        h[i] = -1, in[i] = out[i] = belong[i] = 0;    edgenum = 0;}void add(int f,int t){    edge[edgenum] = node(t,h[f]);    h[f] = edgenum++;}void pred(int u,int deep,int pre){    d[u] = deep;    for(int i = h[u]; ~i; i = edge[i].next)    {        //cout << "this" << endl;        int v = edge[i].to;        if(v != pre)            pred(v,deep+1,u);    }}bool Find(int u){    for(int i = h[u] ; ~i; i = edge[i].next)    {        int v = edge[i].to;        if(used[v]) continue;        used[v] = 1;        if(!belong[v] || Find(belong[v]))        {            belong[v] = u;            return true;        }    }    return false;}void solve(){    int ans = 0;    for(int i = 1; i <= n; i++)    {        if(d[i]%2==0)        {            memset(used,0,sizeof(used));            if(Find(i)) ans++;        }    }    printf("%d\n",ans);}int main(){    int a,b,c;    while(~scanf("%d",&n))    {        init();        for(int i = 0; i < n; i++)        {            scanf("%d:(%d)",&a,&b);            a++;            for(int j = 0; j < b ; j++)            {                scanf("%d",&c); c++;                add(a,c);   add(c,a);                out[a]++;   in[c]++;            }        }        int root = -1;        for(int i = 1; i <= n; i++)            if(!in[i]) {root = i;break;}        pred(root,0,-1);        solve();    }    return 0;}