C - Path Graph? https://atcoder.jp/contests/abc287/tasks/abc287_c

abc287_c.py

import sys

sys.setrecursionlimit(10**7)


def solv():
    if N - 1 != M:
        return False
    for i in range(N):
        if len(G[i]) > 2:
            return False
    visited = [False] * N
    dfs(0, visited)
    return all(visited)


def dfs(v, visited):
    visited[v] = True
    for u in G[v]:
        if visited[u]:
            continue
        dfs(u, visited)
    return


N, M = map(int, input().split())
G = [[] for _ in range(N)]
for _ in range(M):
    u, v = map(lambda x: int(x) - 1, input().split())
    G[u].append(v)
    G[v].append(u)
print("Yes") if solv() else print("No")