Swapping letters

import queue

n = int(input())
forbidden = set()
for i in range(n):
    tup = input().split()
    forbidden.add(tup[0]+tup[1])
    forbidden.add(tup[1]+tup[0])
x = input()
y = input()

left = queue.Queue()
left.put(x)
visited = set()

while not left.empty():
    current = left.get()
    #print(current)
    if current == y:
        break
    if current not in visited:
        for i in range(len(current)-1):
            if current[i]+current[i+1] not in forbidden:
                nex = current[:i]+current[i+1]+current[i]+current[i+2:]
                left.put(nex)
        visited.add(current)

if current == y:
    print("YES")
else:
    print("NO")

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: RUNTIME ERROR

Test 5

Verdict: RUNTIME ERROR

Test 6

Verdict: ACCEPTED

Test 7

Verdict: RUNTIME ERROR

Test 8

Verdict: RUNTIME ERROR

Test 9

Verdict: RUNTIME ERROR

Test 10

Verdict: RUNTIME ERROR