<pre name="code" class="cpp"># include <bits/stdc++.h>
using namespace std;
typedef struct node
{
    int to;// u--->v的有向邊記錄點v
    struct node*next;
} Node;

Node*List[11];
int indegree[11];
int Stack[11];//入度為0的頂點入棧
void TopSort(int n,int m);
int main()
{
    int n,m,i;
    int a,b;
    Node *p;
    while(cin>>n>>m)
    {
        if(n == 0 && m == 0)
        {
            break;
        }
        memset(List,0,sizeof(List));
        memset(indegree,0,sizeof(indegree));
        while(m--)
        {
            //scanf("%d%d",&a,&b);
            cin>>a>>b;
            indegree[b]++; //頂點b入度加1
            p = (Node*)malloc(sizeof(Node));
            p->to = b;
            p->next = NULL;
            if(List[a] == NULL)//邊連結串列上沒有節點
            {
                List[a] = p;
            }
            else//逆序建表
            {
                /*注意開的是結構體指標陣列*/
               p->next = List[a];
               List[a] = p;
            }
        }
        TopSort(n,m);
        //釋放邊連結串列上各點
        for(i=1;i<=n;i++)
        {
            p = List[i];
            while(p!=NULL)
            {
                List[i] = p->next;
                free(p);
                p = List[i];
            }
        }
    }
}

void TopSort(int n,int m)
{
    int i,j,k;
    int top = -1;
    Node*tmp;
    bool bcycle = false;//是否存在有向環,初始認為沒有
    for(i=1;i<=n;i++)
    {
        if(indegree[i] == 0)
        {
            Stack[++top] = i;
        }
    }
    for(i=1;i<=n;i++)
    {
        if(top == -1)//如果在頂點全部輸出之前棧頂指標top已變成了-1,表明棧已空.已經沒有入度為0的點了,說明存在有向環
        {
            bcycle = true;
            break;
        }
        else
        {
            j = Stack[top--];
            tmp = List[j];
            //遍歷頂點j的邊連結串列,每條邊的終點入度減去1
            while(tmp!=NULL)
            {
                k = tmp->to;
                if(--indegree[k] == 0)//入度為0進棧
                {
                    Stack[++top] = k;
                }
                tmp = tmp->next;
            }
        }
    }
    if(bcycle)
    {
        printf("NO\n");
    }
    else
    {
        printf("YES\n");
    }
}