Correct structure of related data in the API method - reactjs

Question

Good afternoon!
I'm developing an API for a web service. I need to render the form with default data downloaded from server.
The method 'small-form' returns dictionaries of countries and cities.
Every city has an id of the country where it is located. If we need to know all cities in the country - it is necessary to iterate through all the cities.
There are two options to solve this problem:
1.Add an list of id's with all cities related to the countries.
2.Create a separate dictionary, where the keys are id's of the countries, and the values ​​are the lists of cities id's.
I can not understand how will it be more correct:
{
"Countries": {
"country id": {
"id": "country id",
"name": "country name",
"citys": ["city id", "city id" ...]
},
},
"Cities": {
"city id": {
"id": "id of the city",
"name": "city name",
"country__id": "country id",
}
}
}
or so:
{
"Countries": {
"country id": {
"id": "country id",
"name": "country name",
},
},
"Cities": {
"city id": {
"id": "id of the city",
"name": "city name",
"country_id": "country id",
}
},
"countries_cities":{
"country id":["id city", "city"...]
}
}