Get the list of record ids from Salesforce sobject records.

list_id.md

Common way

List<Contact> contacts = [Select Id, Name from Contact];
List<Id> contactIds = new List<Id>();
for(Contact c: contacts){
    contactIds.add(c.Id);
}

One line only

List<Contact> contacts = [Select Id, Name from Contact];
List<Id> contactIds = new List<Id>(new Map<Id, Contact>(contacts).keySet());