Problem: You are given an undirected unweighted graph with N vertices and M edges that contains neither self-loops nor double edges.   Here, a _self-loop_ is an edge where a_i = b_i (1≤i≤M), and _double edges_ are two edges where (a_i,b_i)=(a_j,b_j) or (a_i,b_i)=(b_j,a_j) (1≤i<j≤M).   How many different paths start from vertex 1 and visit all the vertices exactly once?   Here, the endpoints of a path are considered visited.  For example, let us assume that the following undirected graph shown in Figure 1 is given.    Figure 1: an example of an undirected graph  The following path shown in Figure 2 satisfies the condition.    Figure 2: an example of a path that satisfies the condition  However, the following path shown in Figure 3 does not satisfy the condition, because it does not visit all the vertices.    Figure 3: an example of a path that does not satisfy the condition  Neither the following path shown in Figure 4, because it does not start from vertex 1.    Figure 4: another example of a path that does not satisfy the condition

Submission attempts (in order):
N, M = map(int, input().split())

l = []

for i in range(N):
    l.append(list(map(int, input().split())))

lis = [[] for i in range(N)]

for i in range(N):
    lis[l[i][0] - 1].append(l[i][1])
    lis[l[i][1] - 1].append(l[i][0])

print(lis)


# list = [[0 for c in range(M)] for r in range(N)]
#

#     tmp_n, tmp_m = map(int, input().split())
#     list[tmp_n - 1][tmp_m - 1] = 1
#
#

#     results = [[0 for c in range(M)] for r in range(N)]

#         print(i,j)
#     return results




# def travel(now, nextnodes, visited):
#     ends.append(visited)
#     visited.append(now + 1)
#     for node in nextnodes:
#         if node in visited:
#             continue
#         travel(node - 1, lis[node - 1], visited[:])
#
#
# travel(0, lis[0], [])

que = [[0, lis[0], []]]
ends = []
while que:
    now, nextnodes, visited = que.pop()
    ends.append(visited)
    visited.append(now + 1)
    for node in nextnodes:
        if node in visited:
            continue
        que.append([node - 1, lis[node - 1], visited[:]])

print(len(list(filter(lambda i: len(i) is 3, ends))))

N, M = map(int, input().split())

l = []

for i in range(N):
    l.append(list(map(int, input().split())))

lis = [[] for i in range(N)]

for i in range(N):
    lis[l[i][0] - 1].append(l[i][1])
    lis[l[i][1] - 1].append(l[i][0])

print(lis)


# list = [[0 for c in range(M)] for r in range(N)]
#

#     tmp_n, tmp_m = map(int, input().split())
#     list[tmp_n - 1][tmp_m - 1] = 1
#
#

#     results = [[0 for c in range(M)] for r in range(N)]

#         print(i,j)
#     return results




# def travel(now, nextnodes, visited):
#     ends.append(visited)
#     visited.append(now + 1)
#     for node in nextnodes:
#         if node in visited:
#             continue
#         travel(node - 1, lis[node - 1], visited[:])
#
#
# travel(0, lis[0], [])

que = [[0, lis[0], []]]
ends = []
while que:
    now, nextnodes, visited = que.pop()
    ends.append(visited)
    visited.append(now + 1)
    for node in nextnodes:
        if node in visited:
            continue
        que.append([node - 1, lis[node - 1], visited[:]])

print(len(list(filter(lambda i: len(i) is 3, ends))))


N, M = map(int, input().split())

l = []

for i in range(M):
    l.append(list(map(lambda x: int(x) - 1, input().split())))

lis = [[] for i in range(N)]

for i in range(M):
    lis[l[i][0]].append(l[i][1])
    lis[l[i][1]].append(l[i][0])



# list = [[0 for c in range(M)] for r in range(N)]
#

#     tmp_n, tmp_m = map(int, input().split())
#     list[tmp_n - 1][tmp_m - 1] = 1
#
#

#     results = [[0 for c in range(M)] for r in range(N)]

#         print(i,j)
#     return results




# def travel(now, nextnodes, visited):
#     ends.append(visited)
#     visited.append(now + 1)
#     for node in nextnodes:
#         if node in visited:
#             continue
#         travel(node - 1, lis[node - 1], visited[:])
#
#
# travel(0, lis[0], [])

que = [[0, lis[0], []]]
ends = []

while que:
    now, nextnodes, visited = que.pop()
    ends.append(visited)
    visited.append(now)
    for node in nextnodes:
        if node in visited:
            continue
        que.append([node, lis[node], visited[:]])

print(len(list(filter(lambda ii: len(ii) is N, ends))))


Verdicts: Runtime Error

Runtime Error

Accepted