Proving Equivalences UVALive

就是统计入度为0 的点 和 出度为0 的点  输出 大的那一个，，

若图中只有一个强连通分量 则输出0即可

和https://www.cnblogs.com/WTSRUVF/p/9301096.html  这题差不多  poj1236

#include <iostream>
#include <cstdio>
#include <sstream>
#include <cstring>
#include <map>
#include <set>
#include <vector>
#include <stack>
#include <queue>
#include <algorithm>
#include <cmath>
#define rap(a, n) for(int i=a; i<=n; i++)
#define MOD 2018
#define LL long long
#define ULL unsigned long long
#define Pair pair<int, int>
#define mem(a, b) memset(a, b, sizeof(a))
#define _  ios_base::sync_with_stdio(0),cin.tie(0)
//freopen("1.txt", "r", stdin);
using namespace std;
const int maxn = 20005, INF = 0x7fffffff;
vector<int> G[maxn];
int pre[maxn], lowlink[maxn], sccno[maxn], dfs_clock, scc_cnt, in[maxn], out[maxn];
stack<int> s;

void dfs(int u)
{
    pre[u] = lowlink[u] = ++dfs_clock;
    s.push(u);
    for(int i=0; i<G[u].size(); i++)
    {
        int v = G[u][i];
        if(!pre[v])
        {
            dfs(v);
            lowlink[u] = min(lowlink[u], lowlink[v]);
        }
        else if(!sccno[v])
            lowlink[u] = min(lowlink[u], pre[v]);
    }
    if(lowlink[u] == pre[u])
    {
        scc_cnt++;
        for(;;)
        {
            int x = s.top(); s.pop();
            sccno[x] = scc_cnt;
            if(x == u) break;
        }
    }
}

void init()
{
    dfs_clock = scc_cnt = 0;
    mem(sccno, 0);
    mem(pre, 0);
    mem(out, 0);
    mem(in, 0);
    for(int i=0; i<maxn; i++) G[i].clear();
}


int main()
{
    int T, n, m;
    scanf("%d", &T);
    while(T--)
    {
        init();
        scanf("%d%d", &n, &m);
        rap(1, m)
        {
            int u, v;
            scanf("%d%d", &u, &v);
            G[u].push_back(v);
   //         G[v].push_back(u);
        }
        rap(1, n)
            if(!pre[i])
                dfs(i);
        rap(1, n)
        {
            for(int j=0; j<G[i].size(); j++)
            {
                if(sccno[i] != sccno[G[i][j]])
                    out[sccno[i]]++, in[sccno[G[i][j]]]++;
            }
        }
        if(scc_cnt == 1)
        {
            cout<< '0' <<endl;
            continue;
        }
        int a = 0, b = 0;
        rap(1, scc_cnt)
        {
            if(!in[i])
                a++;
            if(!out[i])
                b++;
        }
        cout<< max(a, b) <<endl;

    }

    return 0;
}