dfs + adjacency list

def dfs(h):
    used[h] = True
    for e in g[h]:
        if not used[e]:
            dfs(e)


n, m = map(int, input().split())
g = [[] for i in range(n)]
for _ in range(m):
    x, y = map(int, input().split())
    x -= 1
    y -= 1
    g[x].append(y)
    g[y].append(x)
used = [False] * n
dfs(0)