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

先用Tarjan求出强连通分量，其他算法也可以的，再缩点，然后从入度为0的点开始暴搜，如果深度达到强连通分量的个数即可行。这一题一定要注意一点：我用cin的时候TLE，该scanf就好了。。。

#include <cstdio>
#include <cstring>
#include <vector>
using namespace std;

const int maxN = 1000 + 10;
vector<int> edge[maxN];
int low[maxN];
int stack[maxN],top;
int del[maxN];
int DFN[maxN];
int ComNum;
int cnt;

bool flag;
bool in[maxN];
vector<int> sedge[maxN];

void Tarjan(int x)
{
    stack[top ++] = x;
    low[x] = DFN[x] = cnt ++;

    int len = edge[x].size();
    for(int i = 0;i < len;i ++)
    {
        int u = edge[x][i];
        if(!DFN[u])
        {
            Tarjan(u);
            if(low[x] > low[u]) low[x] = low[u];
        }
        else if(!del[u])
        {
            if(low[x] > DFN[u]) low[x] = DFN[u];
        }
    }

    if(DFN[x] == low[x])
    {
        ComNum ++;
        do
        {
            del[stack[top-1]] = ComNum;
        }while(stack[--top] != x);
    }
}

void UseTarjan(int n)
{
    memset(DFN,0,sizeof(DFN));
    memset(del,0,sizeof(del));
    ComNum = 0;
    top = 0;
    cnt = 1;
    for(int i = 1;i <= n;i ++)
    {
        if(!DFN[i])
        {
            Tarjan(i);
        }
    }
}

void dfs(int k,int cc)
{
    in[k] = true;
    //cc ++;
    if(cc == ComNum)
    {
        flag = true;
        return;
    }
    int len = sedge[k].size();
    for(int i = 0;i < len;i ++)
    {
        if(!in[sedge[k][i]])
        {
            dfs(sedge[k][i],cc+1);
        }
        if(flag)
        return;
    }
}

int main()
{
    int n,m;
    int cases;
    scanf("%d",&cases);
    while(cases --)
    {
        scanf("%d%d",&n,&m);
        for(int i = 0;i <= n;i ++)
        {
            edge[i].clear();
            sedge[i].clear();
        }

        for(int i = 0;i < m;i ++)
        {
            int u,v;
           scanf("%d%d",&u,&v);
            edge[u].push_back(v);
        }
        UseTarjan(n);
        if(ComNum == 1)
        {
            printf("Yes\n");
            continue;
        }
        int i = 1;
        memset(in,false,sizeof(in));
        for(;i <= n;i ++)
        {
            int len = edge[i].size();
            for(int j = 0;j < len;j ++)
            {
                if(del[i] != del[edge[i][j]])
                {
                    in[del[edge[i][j]]] = true;
                    sedge[del[i]].push_back(del[edge[i][j]]);//将缩点构成图
                }
            }
        }
        int t = 0;
        int k = 0;
        for(i = 1;i <= ComNum;i ++)
        {
            if(!in[i])//寻找入度为0的点
            {
                t ++;
                k = i;//记录下入度为0的点
            }
        }
        if(t > 1)//如果入度为0的点超过一个这几个点无法到达
        {
            printf("No\n");
            continue;
        }

        flag = false;
        memset(in,false,sizeof(in));
        dfs(k,1);//从入度为0的点开始搜
        if(flag)
        {
            printf("Yes\n");
        }
        else
        {
            printf("No\n");
        }
    }

    return 0;
}