[BOJ/백준] 4803 트리

import sys
input = sys.stdin.readline
def check(num):
    visited[num] = True
    stack = [num]
 
    while stack:
        node = stack.pop(0)
 
        for next_node in tree[node]:
            if tree[node][next_node] == 1:
                if not visited[next_node]:
                    visited[next_node] = True
                    stack.append(next_node)
                    tree[node][next_node] = 0
                    tree[next_node][node] = 0
                else:
                    return False
    return True
            
 
 
tc = 1
while True:
    N,M = map(int,input().split())
    if N+M == 0:
        break
    parent_cnt = [0]*(N+1)
    tree = [{} for _ in range(N+1)]
    for _ in range(M):
        x,y = map(int,input().split())
        tree[x][y] = 1
        tree[y][x] = 1
    cnt = 0
    visited = [False]*(N+1)
    for num in range(1,N+1):
        if not visited[num]:
            if check(num):
                cnt += 1
    if cnt == 0:
        print(f'Case {tc}: No trees.')
    elif cnt == 1:
        print(f'Case {tc}: There is one tree.')
    else:
        print(f'Case {tc}: A forest of {cnt} trees.')
 
 
    tc += 1