题意 ： 给你m个二元关系， 问是否可以确定各个节点的先后关系；

思路： 拓扑排序， 判断是否有环；

#include
     
      using namespace std;const int maxn = 1e4 + 131;struct Node{    int nxt, to;}edge[maxn];int Head[maxn], tot;void AddEdge(int From, int To){    edge[tot].to = To;    edge[tot].nxt = Head[From];    Head[From] = tot++;}map
      
        StoI;string a, b;int Vis[maxn];/// 记录节点的访问次数void TopSort(int n){    queue
       
         qu;    int lest = n;    for(int i = 1; i <= n; ++i)    {        if(!Vis[i]) qu.push(i);    }    while(!qu.empty())    {        int u = qu.front();        qu.pop();        --lest;        for(int i = Head[u]; i != -1; i = edge[i].nxt)        {            int v = edge[i].to;            --Vis[v];            if(!Vis[v]) qu.push(v);        }    }    if(lest) puts("No");    else puts("Yes");}int main(){    int t;    scanf("%d",&t);    for(int kase = 1; kase <= t; ++kase)    {        int m;        scanf("%d",&m);        StoI.clear();        memset(Head,-1,sizeof(Head));        tot = 0;        int n = 0;        memset(Vis, 0, sizeof(Vis));        for(int i = 1; i <= m; ++i)        {            cin >> a >> b;            if(StoI.find(a) == StoI.end())                StoI[a] = ++n;            if(StoI.find(b) == StoI.end())                StoI[b] = ++n;            ++Vis[StoI[b]];            AddEdge(StoI[a], StoI[b]);        }        printf("Case %d: ",kase);        TopSort(n);    }    return 0;}