Problem p02834 (python)

Fixed submission:
import sys
input = sys.stdin.readline

n,u,v = map(int,input().split())
edges = [[] for _ in range(n)]

for _ in range(n-1):
    a,b = map(int,input().split())
    edges[a-1].append(b-1)
    edges[b-1].append(a-1)

visit1 = [False] * n
visit2 = [False] * n
visit1[u-1] = True
visit2[v-1] = True

dis1 = [-1] * n
dis2 = [-1] * n
dis1[u-1] = 0
dis2[v-1] = 0

stack1 = [u-1]
stack2 = [v-1]

while stack1:
    l = stack1.pop(-1)
    for i in edges[l]:
        if visit1[i] == False:
            visit1[i] = True
            dis1[i] = dis1[l] + 1
            stack1.append(i)

while stack2:
    l = stack2.pop(-1)
    for i in edges[l]:
        if visit2[i] == False:
            visit2[i] = True
            dis2[i] = dis2[l] + 1
            stack2.append(i)

ans = 0

for i in range(n):
    if dis1[i] < dis2[i]:
        ans = max(ans, dis2[i])

print(ans-1)

Buggy submission (verdict: Runtime Error):
n,u,v = map(int,input().split())

edges = [[] for _ in range(n)]

for _ in range(n-1):
    a,b = map(int,input().split())
    edges[a-1].append(b-1)
    edges[b-1].append(a-1)

visit1 = [False]*n
visit2 = [False]*n

visit1[u-1] = True
visit2[v-1] = True

stack1 = [u-1]
stack2 = [v-1]

dis1 = [-1] * n
dis2 = [-1] * n
dis1[u-1] = 0
dis2[v-1] = 0

while stack1:
    for _ in range(len(stack1)):
        q1 = stack1.pop(-1)
        for i in edges[q1]:
            if not visit1[i] and not visit2[i]:
                visit1[i] = True
                dis1[i] = dis1[q1] + 1
                stack1.append(i)

    for _ in range(len(stack2)):
        q2 = stack2.pop(-1)
        for i in edges[q2]:
            if not visit2[i]:
                visit2[i] = True
                dis2[i] = dis2[q2] + 1
                stack2.append(i)

while stack2:
    q = stack2.pop(-1)
    for i in edges[q]:
        if not visit2[i]:
            visit2[i] = True
            dis2[i] = dis2[q] + 1
            stack2.append(i)

Ans = 0
for i in range(n):
    If dis1[i] != 0:
        Ans =max(Ans,dis2[i])
ans = Ans -1

print(ans)

Buggy submission (verdict: Runtime Error):
n,u,v = map(int,input().split())

edges = [[] for _ in range(n)]

for _ in range(n-1):
    a,b = map(int,input().split())
    edges[a-1].append(b-1)
    edges[b-1].append(a-1)

visit1 = [False]*n
visit2 = [False]*n

visit1[u-1] = True
visit2[v-1] = True

stack1 = [u-1]
stack2 = [v-1]

dis1 = [-1] * n
dis2 = [-1] * n
dis1[u-1] = 0
dis2[v-1] = 0

while stack1:
    for _ in range(len(stack1)):
        q1 = stack1.pop(-1)
        for i in edges[q1]:
            if not visit1[i] and not visit2[i]:
                visit1[i] = True
                dis1[i] = dis1[q1] + 1
                stack1.append(i)

    for _ in range(len(stack2)):
        q2 = stack2.pop(-1)
        for i in edges[q2]:
            if not visit2[i]:
                visit2[i] = True
                dis2[i] = dis2[q2] + 1
                stack2.append(i)

while stack2:
    q = stack2.pop(-1)
    for i in edges[q]:
        if not visit2[i]:
            visit2[i] = True
            dis2[i] = dis2[q] + 1
            stack2.append(i)

Ans = 0
for I in range(n):
    If dis1[I] != 0:
    Ans =max(Ans,dis2[i])
ans = Ans -1

print(ans)