poj 1236 Network of Schools 【Tarjan】

题目链接：http://poj.org/problem?id=1236

题意：

本题为有向图。
需解决两个问题：
1 需要给多少个点，才能传遍所有点。
2 加多少条边，使得整个图变得强连通。
使用Tarjan进行缩点，得到一个SCC图、
这个图有多少个入度为0的，多少个出度为0的。
假设有n个入度为0，m个出度为0
那么第一个答案就是n,第二个答案是max(n,m)
代码：

#include <stdio.h>
#include <iostream>
#include <math.h>
#include <stdlib.h>
#include <ctype.h>
#include <algorithm>
#include <vector>
#include <string.h>
#include <string>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <sstream>
#include <time.h>

using namespace std;

const int MAXN = 20010;
const int MAXM = 50010;

struct Edge
{
    int to, next;
}edge[MAXM];

int head[MAXM], tot;
int Low[MAXN], Dfn[MAXN], Stack[MAXN], Belong[MAXN];//Belong的值为 1 ~ scc
int Index, top;
int scc;//强连通个数
bool Instack[MAXN];
int num[MAXN];//各个强连通包含的点的个数

void addedge(int u, int v)
{
    edge[tot].to = v;
    edge[tot].next = head[u];
    head[u] = tot++;
}

void Tarjan(int u)
{
    int v;
    Low[u] = Dfn[u] = ++Index;
    Stack[top++] = u;
    Instack[u] = true;
    for (int i = head[u]; i != -1; i = edge[i].next)
    {
        v = edge[i].to;
        if (!Dfn[v])
        {
            Tarjan(v);
            if (Low[u] > Low[v]) 
                Low[u] = Low[v];
        }
        else if (Instack[v] && Low[u] > Dfn[v])
            Low[u] = Dfn[v];
    }
    if (Low[u] == Dfn[u])
    {
        scc++;
        do
        {
            v = Stack[--top];
            Instack[v] = false;
            Belong[v] = scc;
            num[scc]++;
        } while (v != u);
    }
}

int in[MAXN], out[MAXN];

void solve(int N)
{
    memset(Dfn,0,sizeof(Dfn));
    memset(Instack,false,sizeof(Instack));
    memset(num,0,sizeof(num));
    Index = scc = top = 0;
    for (int i = 1; i <= N; i++)
    {
        if (!Dfn[i])
            Tarjan(i);
    }
    if (scc == 1)
    {
        printf("1\n0\n");
        return;
    }
    for (int i = 1; i <= scc; i++)
        in[i] = out[i] = 0;
    for (int u = 1; u <= N; u++)
    {
        for (int i = head[u]; i != -1; i = edge[i].next)
        {
            int v = edge[i].to;
            if (Belong[u] != Belong[v])
            {
                in[Belong[v]]++;
                out[Belong[u]]++;
            }
        }
    }
    int ans1 = 0, ans2 = 0;
    for (int i = 1; i <= scc; i++)
    {
        if (in[i] == 0) ans1++;
        if (out[i] == 0) ans2++;
    }
    //printf("%d\n",scc);
    printf("%d\n%d\n",ans1,max(ans1,ans2));
}

void init()
{
    tot = 0;
    memset(head,-1,sizeof(head));
}

int main()
{
    int n;
    int u, v;
    while (~scanf("%d", &n))
    {
        init();
        for (int i = 1; i <= n; i++)
        {
            while (~scanf("%d", &u) && u)
            {
                addedge(i,u);
            }
        }
        solve(n);
    }
    return 0;
}