nyoj 120 校园网络

题目链接：http://acm.nyist.net/JudgeOnline/problem.php?pid=120

思路：先将原图强连通缩点为新图，统计新图中入度，出度为0的点的个数，两者取最大值即为答案。

代码如下：

#include "stdio.h"
#include "string.h"
#include "stack"
#include "vector"
using namespace std;

#define N 105
#define M 10005

struct node
{
    int x,y;
    int next;
}edge[2*M];
int idx,head[N];
void Init(){ idx=0; memset(head,-1,sizeof(head)); }
inline int MIN(int a,int b){ return a<b?a:b; }
inline int MAX(int a,int b){ return a>b?a:b; }
void Add(int x,int y)
{
    edge[idx].x=x;
    edge[idx].y=y;
    edge[idx].next=head[x];
    head[x]=idx++;
}

bool vis[N];
int dfn[N],low[N];
int dfs_clock;
int countt;
int belong[N];
stack<int> q;

void DFS(int x)   //强连通缩点算法
{
    int i,y;
    q.push(x);
    vis[x] = true;
    dfn[x] = low[x] = ++dfs_clock;
    for(i=head[x]; i!=-1; i=edge[i].next)
    {
        y = edge[i].y;
        if(!dfn[y])
        {
            DFS(y);
            low[x] = MIN(low[x],low[y]);
        }
        else if(vis[y])
            low[x] = MIN(dfn[y],low[x]);
    }
    if(low[x]==dfn[x])
    {
        int temp;
        countt++;
        while(1)
        {
            temp = q.top();
            q.pop();
            belong[temp] = countt;
            vis[temp] = false;
            if(temp==x) break;
        }
    }
}

int ru_du[N],chu_du[N];

int Solve(int n)
{
    int i;
    int x,y;
    dfs_clock = countt = 0;
    memset(dfn,0,sizeof(dfn));
    memset(vis,false,sizeof(vis));
    memset(belong,0,sizeof(belong));
    for(i=1; i<=n; ++i)
    {
        if(!dfn[i])
            DFS(i);
    }
    if(countt==2) return 0;  //该图原本就是强连通图，return 0;
    memset(ru_du,0,sizeof(ru_du));
    memset(chu_du,0,sizeof(chu_du));
    for(i=0; i<idx; ++i)
    {
        x = belong[edge[i].x];
        y = belong[edge[i].y];
        if(x!=y)  //统计入度，出度
        {
            chu_du[x]++;
            ru_du[y]++;
        }
    }
    int ru_0=0,chu_0=0;
    for(i=1; i<countt; ++i)
    {
        if(ru_du[i]==0) ru_0++;
        if(chu_du[i]==0) chu_0++;
    }
    int ans = MAX(ru_0,chu_0); //return 入度，出度最大的那个数
    return ans;
}

int main()
{
    int T;
    int n;
    int i,k;
    scanf("%d",&T);
    while(T--)
    {
        Init();
        scanf("%d",&n);
        for(i=1; i<=n; ++i)
        {
            while(scanf("%d",&k),k)
                Add(i,k);
        }
        printf("%d\n",Solve(n));
    }
    return 0;
}