[BOJ/백준] 2458 키순서

import sys
import collections
input = sys.stdin.readline

def bfs1(x):
    que=collections.deque()
    que.append(x)
    temp=set()
    temp.add(x)
    while que:
        node=que.popleft()
        for i in graph1[node]:
            if i not in temp:
                temp.add(i)
                visit[x]+=1       
                que.append(i)
    return
def bfs2(x):
    que=collections.deque()
    que.append(x)
    temp=set()
    temp.add(x)
    while que:
        node=que.popleft()
        for i in graph2[node]:            
            if i not in temp:
                temp.add(i)
                visit[x]+=1       
                que.append(i)
    return
 

N,M = map(int,input().split())
arr=[list(map(int,input().split())) for i in range(M)]
graph1=[[] for i in range(N+1)]
graph2=[[] for i in range(N+1)]
for i in arr:
    graph1[i[0]].append(i[1])
    graph2[i[1]].append(i[0])
visit=[0]*(N+1)
for i in range(1,N+1):        
    bfs1(i)
    bfs2(i)

cnt=visit.count(N-1)
print(cnt)