poj 2762 Going from u to v or from v to u?

缩点 + 拓扑排序，每次判断入度为0的点有几个，2个或两个以上就return false，然后一种特殊情况就是缩点之后就剩下一个点，直接return true，开始因为没考虑到这个居然wa了一次！我日！！

poj已经阻止不了我了。。╮(╯▽╰)╭

#include<iostream>
#include<vector>
#include<algorithm>
#include<cstdio>
#include<queue>
#include<stack>
#include<string>
#include<map>
#include<set>
#include<cmath>
#include<cassert>
#include<cstring>
#include<iomanip>
using namespace std;

#ifdef _WIN32
#define i64 __int64
#define out64 "%I64d\n"
#define in64 "%I64d"
#else
#define i64 long long
#define out64 "%lld\n"
#define in64 "%lld"
#endif

#define FOR(i,a,b)      for( int i = (a) ; i <= (b) ; i ++)
#define FF(i,a)         for( int i = 0 ; i < (a) ; i ++)
#define FFD(i,a)        for( int i = (a)-1 ; i >= 0 ; i --)
#define S64(a)          scanf(in64,&a)
#define SS(a)           scanf("%d",&a)
#define LL(a)           ((a)<<1)
#define RR(a)           (((a)<<1)+1)
#define SZ(a)           ((int)a.size())
#define PP(n,m,a)		puts("---");FF(i,n){FF(j,m)cout << a[i][j] << ' ';puts("");}
#define pb              push_back
#define CL(Q)           while(!Q.empty())Q.pop()
#define MM(name,what)   memset(name,what,sizeof(name))
#define read            freopen("in.txt","r",stdin)
#define write           freopen("out.txt","w",stdout)

const int inf = 0x3f3f3f3f;
const i64 inf64 = 0x3f3f3f3f3f3f3f3fLL;
const double oo = 10e9;
const double eps = 10e-10;
const double pi = acos(-1.0);
const int maxn = 1011;

vector<int>g[maxn];
vector<int>v[maxn];
vector<int>back[maxn];
vector<int>s;
int dfn[maxn];
int low[maxn];
int be[maxn];
bool ins[maxn];
bool hash[maxn][maxn];
int df,nb;
int n,m,T;
int in[maxn];
int out[maxn];
bool vis[maxn];

void tarjan(int now)
{
    int to;
    dfn[now] = low[now] = df++;
    s.pb(now);
    ins[now] = true;
    FF(i,g[now].size())
    {
        to = g[now][i];
        if(!dfn[to])
        {
            tarjan(to);
            low[now] = min(low[now],low[to]);
        }
        else if(ins[to])
        {
            low[now] = min(low[now],low[to]);
        }
    }

    if(low[now] == dfn[now])
    {
        while(s.back() != now)
        {
            to = s.back();
            ins[to] = false;
            s.pop_back();
            be[to] = nb;
        }
        to = s.back();
        be[to] = nb++;
        s.pop_back();
        ins[to] = false;
    }
    return ;
}


void start()
{
    s.clear();
    MM(dfn,0);
    MM(low,0);
    MM(be,0);
    MM(ins,false);
    MM(hash,false);
    df = nb =1;
    MM(in,0);
    MM(out,0);
    for(int i=1;i<=n;i++)
    {
        if(!dfn[i])
        {
            tarjan(i);
        }
    }
    nb--;
    int from,to;
    FOR(i,1,n)
    {
        from = i;
        FF(j,g[i].size())
        {
            to = g[i][j];
            if(from != to && !hash[from][to] )
            {
                v[from].pb(to);
                back[to].pb(from);
                in[to]++;
                out[from]++;
                hash[from][to] = true;
            }
        }
    }
    return ;
}

bool can()
{
    start();
    int num;
    int temp;
    MM(vis,false);
    int from,to;
    if(nb==1)
    {
        return true;
    }
    while(true)
    {
        num = 0;
        FOR(i,1,nb)
        {
            if(!vis[i] && !in[i])
            {
                vis[i] = true;
                num++;
                temp = i;
            }
        }
        if(num >= 2)
        {
            return false;
        }
        else if(num == 1)
        {
            FF(i,v[temp].size())
            {
                to = v[temp][i];
                in[to] --;
            }
        }
        else
        {
            break;
        }
    }
    FOR(i,1,nb)
    {
        if(!vis[i])
        {
            return false;
        }
    }
    return true;
}

int main()
{
    cin>>T;
    while(T--)
    {
        cin>>n>>m;
        FOR(i,1,n)
        {
            g[i].clear();
            v[i].clear();
            back[i].clear();
        }
        int now,to;
        for(int i=1;i<=m;i++)
        {
            cin>>now>>to;
            g[now].push_back(to);
        }
        if(can())
        {
            cout<<"Yes"<<endl;
        }
        else
        {
            cout<<"No"<<endl;
        }
    }
    return 0;
}