문제 : https://www.acmicpc.net/problem/2606

▶ 코드 ( 큐 ver. )

from collections import deque

n = int(input())
m = int(input())

s = [[0]*(n+1) for i in range(n+1)]
visited = [0]*(n+1)

for _ in range(m):
    a, b = map(int, input().split())
    s[a][b] = 1
    s[b][a] = 1

def bfs(v):
    dq = deque([v])
    visited[v] = 1
    c = 0
    
    while(dq):
        v = dq.popleft()
        for i in range(1, n+1):
            if visited[i] == 0 and s[v][i] == 1:
                dq.append(i)
                visited[i] = 1
                c += 1
    print(c)


bfs(1)