Help me in solving TOURMAP problem

help
,
1

My issue

for i in range(int(input())):
d={}
s=set()
des=set()
n=int(input())
if n==1:
print(0)
continue
for k in range(n-1):
a,b,c=input().split()
d[a]=[b,int(c)]
s.add(a)
des.add(b)
source=list(s-des)[0]
cost=0
while(source in d):
print(source,d[source][0],d[source][1])
cost+=d[source][1]
source=d[source][0]
print(cost)

can anybody say why it is giving runtime error

My code

for i in range(int(input())):
    d={}
    s=set()
    des=set()
    n=int(input())
    if n==1:
        print(0)
        continue
    for k in range(n-1):
        a,b,c=input().split()
        d[a]=[b,int(c)]
        s.add(a)
        des.add(b)
    source=list(s-des)[0]
    cost=0
    while(source in d):
        print(source,d[source][0],d[source][1])
        cost+=d[source][1]
        source=d[source][0]
    print(cost)

Problem Link: Sridhar Likes Travelling Practice Coding Problem - CodeChef